Why calendars need reforming
Every calendar is a compromise between three numbers that refuse to divide cleanly. The mean solar year is 365.2422 days, not 365. The synodic lunar month is 29.5306 days, not 30. Neither divides into the other without remainder, and neither divides cleanly into the seven-day astronomical week the ancient Babylonians settled on around 600 BCE. Every culture that has tried to track time over more than a few years has had to pick which of these three cycles to lie about, and the history of calendar reform is the history of governments and churches periodically discovering that their preferred lie has accumulated enough error to become embarrassing.
The lunar problem is the most acute. Twelve synodic months come to about 354.37 days, which is roughly eleven days short of the solar year. A pure lunar calendar like the Islamic Hijri calendar simply accepts the drift and lets the months walk through the seasons over a thirty-three year cycle. A lunisolar calendar like the Hebrew calendar or the Chinese calendar inserts a thirteenth intercalary month in seven years out of every nineteen to keep the months and the seasons roughly aligned. A purely solar calendar like the Julian and Gregorian systems abandons the moon entirely and tracks only the Sun, accepting that the months will no longer correspond to lunations and that festivals that were originally lunar (Christian Easter, Jewish Passover) now have to be computed by separate rules layered on top.
The solar problem is more subtle but more consequential. The Earth's orbit is roughly 365.2422 days, and any integer count of days will accumulate error. A year of 365 days drifts forward by about a quarter day annually, which becomes a full day every four years and a full month every 128. A year of 365.25 days drifts backward by about eleven minutes annually, which is the Julian calendar's exact problem and the reason Gregory had to step in. A year of 365.2425 days, the Gregorian compromise, is off by about twenty-six seconds, which becomes a full day only every 3,300 years. The Persian calendar's leap rule is even tighter, off by about one second a year. There is no integer-day rule that is perfect, only ones that are good enough for the political horizon their designers cared about.
Every reform discussed below is a response to one of these three pressures: the church needs Easter on a recognizable date, the state needs tax periods to align with harvests, or a revolutionary government wants to wipe the cultural slate clean and finds out the hard way that the slate refuses to be wiped.
The Julian reform — 46 BCE
The Roman Republic's pre-Julian calendar was an intercalated lunisolar system that had broken down badly by the first century BCE. The Pontifex Maximus, the chief priest, was supposed to insert an intercalary month called Mercedonius into the calendar from time to time to keep the months aligned with the seasons, but the decision was political, the office had been weaponized by factional fighting in the late Republic, and during the civil wars of the 50s and 40s BCE the intercalations had simply stopped happening. By 46 BCE the civil year was running about three months ahead of the actual seasons. Spring festivals were being celebrated in winter. Harvest taxes were due before the harvest.
Julius Caesar, then Pontifex Maximus as well as dictator, commissioned the Greek astronomer Sosigenes of Alexandria to design a replacement. Sosigenes proposed a purely solar year of 365 days plus a single intercalary day every fourth year — the first systematic leap-year rule in the western tradition. To realign the calendar with the seasons before the new system took effect, Caesar declared the year 46 BCE to be 445 days long, inserting two extraordinary intercalary months in addition to the regular Mercedonius. Contemporaries called it the annus confusionis, the year of confusion. The reformed calendar took effect on 1 January 45 BCE and survived essentially unchanged for the next 1,627 years.
The month structure is largely Caesar's. The names of January through June were already in use under the Republic. Quintilis (the fifth month, when counting from March, the original new year) was renamed Julius after his assassination in 44 BCE. Sextilis was renamed Augustus in 8 BCE in honour of his nephew, the first emperor. The familiar story that Augustus took a day from February to give August the same thirty-one days as Julius (so the great-uncle and great-nephew would have months of equal dignity) is repeated everywhere from Wikipedia to school textbooks and is almost certainly false. The earliest attestation of the story is in a thirteenth- century manuscript by Johannes de Sacrobosco, twelve centuries after the events, and the modern reconstruction of the actual Julian month lengths from contemporary sources shows the alternating thirty-one and thirty-day pattern was set by Sosigenes from the start. February was short because it was the last month of the Roman year in the original Republican structure, and short months were always the ones at the end.
The Julian rule of a leap year every fourth year produces an average year length of 365.25 days. That is about eleven minutes too long, and over the centuries the error accumulated. By the time of the Council of Nicaea in 325 CE, the spring equinox had drifted from late March to about 21 March, and the council fixed the date of Easter relative to that drifted equinox. The Julian calendar was now committed to a specific reference point, and as the drift continued the equinox slipped further. By the year 1500 the calendar was off by about ten days, and the problem could no longer be ignored. The Julian to Gregorian converter lets you walk back and forth across the gap for any date.
The Gregorian reform — 1582
The reform project was assembled over decades. The German astronomer Christopher Clavius, working with the Italian physician and amateur calendrical scholar Aloysius Lilius (Luigi Lilio), drafted the technical proposal. Lilius died in 1576, before the work was complete, and his brother delivered the manuscript to the papal commission. The commission, chaired by Cardinal Sirleto and including Clavius and the Spanish theologian Pedro Chacón, refined the proposal and presented it to Pope Gregory XIII, who promulgated it on 24 February 1582 with the bull Inter gravissimas. The bull is held in the Vatican archives and is still consulted as the formal source of the reform.
The reform did three things. First, it cut ten days from the civil calendar of October 1582: in Catholic countries that adopted the bull immediately, Thursday 4 October was followed by Friday 15 October. The choice of October was deliberate — there were few major feasts in those particular days and the agricultural calendar was relatively quiet. Second, it modified the leap-year rule. Under Julius's rule every fourth year was a leap year; under Gregory's, every fourth year is a leap year except century years not divisible by 400. So 1700, 1800, and 1900 were not leap years (despite being divisible by four), but 1600 and 2000 were. This reduces the average year length from 365.25 to 365.2425 days, within twenty-six seconds of the actual tropical year. Third, the reform redefined the algorithm for computing Easter using a new set of epacts and lunar tables, replacing the metonic cycle that the Julian Easter had relied on.
Catholic Europe adopted within weeks. Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of the Italian states followed the bull's date exactly. France adopted on 9 December 1582 (which was followed by 20 December). The Catholic German and Swiss territories adopted in 1583 and 1584. Protestant Europe was a different story. England, the Dutch Republic, the Protestant German states, the Scandinavian countries, and Scotland refused to adopt a reform decreed by the pope, partly out of theological objection and partly out of political rivalry with the Catholic states. For the next 170 years, Europe ran two parallel calendars, with a ten-day gap that widened to eleven days after 1700 (which was not a Julian leap year had the rule been Gregorian but was treated as one by the Julian users).
The adoption schedule across the next two centuries is long. The German Protestant states adopted in 1700. Great Britain and its colonies adopted in 1752. Sweden tried to adopt incrementally between 1700 and 1740, fell off the wagon, and ended up with the unique anomaly of 30 February 1712. Japan adopted in 1873 as part of the Meiji modernization. Greece adopted in 1923. Türkiye adopted in 1926. China adopted civilly in 1912 (although the lunisolar Chinese calendar remained, and still remains, in cultural use). The last major holdout was Saudi Arabia, which adopted the Gregorian calendar for state purposes only in 2016. The Wikipedia article on Gregorian adoption has a sortable table of every country's adoption date.
The English transition of 1752
The English-speaking world adopted the Gregorian calendar through the Calendar (New Style) Act of 1750, sponsored in the House of Lords by Philip Stanhope, the 4th Earl of Chesterfield. By the time the act took effect in September 1752, the Julian calendar had drifted eleven days behind Gregorian (the extra day of drift relative to 1582 being the result of the Julian leap year of 1700, which was not a Gregorian leap year). The act therefore required that Wednesday 2 September 1752 be followed directly by Thursday 14 September 1752, eliminating eleven days from the civil calendar.
The act also changed the start of the legal year. Under the old English convention the year had begun on 25 March, the feast of the Annunciation, which is why historical English documents from before 1752 sometimes date events using a double-year notation like '1 February 1701/1702' — the year was 1701 by the legal calendar but 1702 by the calendar most of the rest of Europe was using. After 1752 the legal year began on 1 January, matching the rest of Europe and matching the de facto practice that English merchants had already adopted in correspondence. A consequence is that the British tax year still begins on 6 April rather than 1 January: when the calendar shifted, the Treasury wanted to keep the same length of year, so it shifted the tax year by eleven days to preserve the tax period, and later by another day to account for the year 1800 not being a leap year. The 6 April British tax year is a direct living artifact of the 1752 reform.
The legend that English mobs took to the streets chanting 'give us our eleven days back' is a literary invention. The image first appears in William Hogarth's 1755 satirical engraving 'An Election Entertainment', which shows a placard with the slogan being carried in a chaotic election rally. Hogarth's print was satirizing the 1754 Oxfordshire election and the political attacks the Tories had made on the Whig government during the passage of the Calendar Act. Robert Poole's 1995 Past and Present article 'Give us our eleven days' established the historical consensus: there are no contemporary newspaper reports of organized public demonstrations against the calendar change, and the mythological riots are at best a vivid back-projection from Hogarth's print into the popular imagination of the next two centuries.
What did happen was a wave of arguments over rent, wages, and contracts. Landlords tried to charge a full month's rent for a short September. Wage labourers paid by the day were short eleven days of pay. The Bank of England moved certain interest payments to preserve the old payment dates. There were court cases. There were petitions. None of this is the same as a riot. The shift was administered, on the whole, with the kind of dull procedural friction that real bureaucratic transitions actually produce, and not with mobs marching on Westminster demanding their lost days.
The most lasting consequence is biographical: George Washington was born on 11 February 1731 under the Old Style calendar, and after 1752 his birthday was retroactively renumbered to 22 February 1732 (gaining both eleven days for the Gregorian shift and a year because the old British calendar had started in March). The US public holiday Washington's Birthday is celebrated on the Gregorian date. Anyone using the Julian-to-Gregorian converter with a date before 1752 in an English source is essentially doing what every serious historian of the early modern Atlantic world has to do constantly.
The Russian Orthodox holdout
Imperial Russia never adopted the Gregorian calendar. From 1582 until the Bolshevik decree of 1918, the Russian state operated on the Julian system, and by the early twentieth century the gap with Western Europe had grown to thirteen days. This is why the October Revolution actually happened in November on the Western calendar. The Bolshevik storming of the Winter Palace took place on 25 October 1917 by the Julian calendar still in civil use in Russia, but that date corresponds to 7 November 1917 by the Gregorian calendar the rest of the world was using. Both dates refer to the same physical day. The revolutionaries themselves kept the name 'October Revolution' for the obvious reason that they did not want to rebrand a foundational event of their movement, and Soviet anniversaries of the revolution continued to be observed on 7 November under the new calendar.
The transition itself came in January 1918. A decree signed by Lenin and dated 24 January (Old Style) declared that the day after 31 January 1918 (Old Style) would be 14 February 1918 (New Style), skipping thirteen days. The decree gave several practical reasons: international correspondence was being mangled by the dual dating, the railway and telegraph systems were having to maintain conversions, and the revolutionary government wanted Russia to function on the same calendar as the European labour movement it hoped would soon overthrow its own bourgeois governments. The Provisional Government had considered the same reform in 1917 but had not pushed it through before being overthrown.
The Russian Orthodox Church refused to follow. The Patriarchate of Moscow held that the Julian calendar was the calendar of the Church Fathers and the Ecumenical Councils, and that switching for the convenience of the state would be to subject the eternal liturgy to the political whim of the day. The position has held to the present. Russian Orthodox Christmas falls on 7 January Gregorian (which is 25 December Julian), and Russian Orthodox Easter is computed by the Julian metonic cycle and almost always falls on a different Sunday from the Catholic and Protestant Easter. The Serbian, Georgian, Jerusalem, and Athonite Orthodox bodies share the same position. The Greek, Romanian, Bulgarian, Antiochian, and Alexandrian Orthodox churches adopted a Revised Julian calendar in 1923 that tracks Gregorian for fixed feasts (so their Christmas is 25 December) but still computes Easter on the Julian cycle. The Russian Orthodox Church Outside Russia (ROCOR), the emigré body that reunited with Moscow in 2007, uses the Old Calendar.
The drift continues to widen. The gap between Julian and Gregorian increases by one day every century year not divisible by 400. The gap was 12 days from 1900 to 2099, will be 14 days from 2100 to 2199, 15 days from 2200, and so on. By the year 2800, Old Style Christmas will fall on 11 January Gregorian. None of this bothers the Patriarchate of Moscow. The schismatic logic that drives the Old Calendarist movement explicitly cites the willingness of the New Calendarists to let the state determine liturgical practice as a sign of compromise with modernity, and the gap is part of the point.
The French Republican calendar — 1793 to 1805
The French Revolution attempted a more radical reform than any of the religious or astronomical adjustments that preceded it. The National Convention adopted the Republican calendar on 5 October 1793, backdating its first day to 22 September 1792, the day after the autumnal equinox of that year and the day the Republic had been proclaimed. The system was designed by a commission that included the mathematicians Joseph-Louis Lagrange and Gaspard Monge, the astronomer Jean-Baptiste Delambre, and the poet Philippe-François-Nazaire Fabre d'Églantine, who was responsible for the most famous and most ridiculed feature: the new month names.
The year had twelve months of exactly thirty days each, divided into three ten-day décades, followed by five (or six in leap years) intercalary days at the end of the year called sansculottides after the working-class revolutionaries who had supported the early Republic. The months were named after natural phenomena appropriate to their season in France: Vendémiaire (the wine harvest), Brumaire (the fog), Frimaire (the frost), Nivôse (the snow), Pluviôse (the rain), Ventôse (the wind), Germinal (the germination), Floréal (the flowers), Prairial (the meadows), Messidor (the harvest), Thermidor (the heat), Fructidor (the fruits). Within each month, the days had agricultural names — almonds, plough, hyacinth, grindstone — replacing the Christian saints' days of the old calendar. Each décade ended with a single day of rest called décadi, which replaced Sunday.
The system was internally consistent and astronomically elegant, but it failed comprehensively as a social institution. The ten-day week meant workers had one day of rest every ten days instead of one every seven, a measurable cut in effective leisure time. The saints' days that had broken up the working calendar were gone, and the décadi rest day was distrusted by Catholic workers as a replacement for Sunday rather than a complement to it. Foreign trade was complicated by the need to translate every date into the Gregorian calendar still in use everywhere else. The month names were mocked in the British press as 'the Wheezy, the Sneezy, the Freezy, the Slippy, the Drippy, the Nippy'. The decimal time experiment that accompanied the reform — ten hours per day, one hundred minutes per hour, one hundred seconds per minute — failed even faster and was abandoned in 1795.
Napoleon Bonaparte abolished the Republican calendar by decree on 9 September 1805, with the reform taking effect on 1 January 1806 (or 11 Nivôse XIV in the old style, the last Republican date used officially). The abolition was partly a gesture of reconciliation with the papacy following the 1801 Concordat and partly a recognition that the system had failed to take root in daily commercial or domestic life. The Paris Commune briefly revived the calendar in 1871, lasting roughly eight weeks before the Commune itself was suppressed. The French Republican calendar converter lets you compute the Republican date for any Gregorian date, both during the original period and as if the calendar had survived to the present.
The Soviet experiment — 1929 to 1940
The Soviet Union ran the most aggressive attack on the seven-day week of the twentieth century, and it failed. The objective was openly anti-religious: a five-day continuous week would eliminate Sunday as a common rest day and break the grip of Christian observance on the workforce. A continuous week would also raise industrial output, since factories could run all five days with one fifth of the workforce resting on each, instead of standing idle for a Sunday.
The decree of 26 August 1929 introduced the Nepreryvka, the 'continuous week' of five days. Workers were assigned to one of five rotating colour groups — yellow, orange, red, violet, green — and each group rested on a different day. A married couple in the yellow group and the red group would never have a day off together. The system was deeply unpopular with workers, who could no longer rely on common family days, attend religious services as communities, or coordinate social life around a shared weekly rhythm. By 1931 the Politburo had abandoned it.
The replacement, introduced in November 1931, was the Shestidnevka, the 'six-day week'. Every month had five fixed rest days, on the 6th, 12th, 18th, 24th, and 30th of each month, with each five-day work period punctuated by a single rest day. The last day of the month (the 30th, or the 1st of certain months in months with thirty-one days) absorbed the rest function in a way that produced an irregular cadence at month boundaries. The Shestidnevka had a fixed weekly rhythm of sorts but completely disregarded the seven-day cycle that the rest of the industrial world used. It also broke Christian Sunday observance, but in a way that was less hostile to family life than the Nepreryvka.
The Shestidnevka was abolished on 26 June 1940 and the seven-day week was restored, with Sunday as the universal rest day. The official reason was industrial coordination with international trading partners. The unofficial reason was that the system had failed: productivity gains were illusory because the disruption of communal rest had degraded morale, family life, and the informal networks of labour discipline that the seven-day cycle had supported. Eleven years of state campaign against the seven-day week had produced a humiliating retreat to the same calendar Lenin had inherited from the tsars. The seven-day week has been the official rhythm of Russian and ex-Soviet civil life ever since.
Ethiopia and Egypt — the thirteen-month calendars
Two of the world's living calendars descend almost unchanged from the ancient Egyptian civil year. The Coptic calendar is the calendar of the Coptic Orthodox Church of Alexandria and of Egyptian Christians more generally. The Ethiopian calendar is the calendar in civil use in Ethiopia and in liturgical use across the Ethiopian and Eritrean Orthodox churches. Both inherit the same fundamental structure: twelve months of exactly thirty days each, plus a short intercalary period at the end of the year. The intercalary period is treated as a thirteenth month, called Epagomenae in Coptic and Pagumē in Ethiopian.
The thirteenth month has five days in a common year and six in a leap year. The leap rule is the original Egyptian one: every fourth year is a leap year, with no Gregorian-style century exception. This means the Coptic and Ethiopian calendars drift against the Gregorian calendar at the same rate that the Julian calendar drifts — about one day every 128 years — but the drift has not historically been a concern because both calendars are anchored in tradition rather than in astronomical correction. The Coptic year begins on 1 Thout, which corresponds to 11 September Gregorian in most years and 12 September in years preceding a Coptic leap year. The Ethiopian year begins on 1 Meskerem, the same date by Gregorian reckoning. The two calendars are essentially synchronized to within their respective religious calendars.
Ethiopia's calendar also differs from the Western reckoning of the year. The current Ethiopian year is roughly seven or eight years behind the Gregorian year, because the Ethiopian Church traditionally dates the Annunciation to a different year from the calculation used by Dionysius Exiguus in the sixth century, which produced the AD numbering of the Western tradition. So while the Gregorian calendar is in the year 2026, the Ethiopian calendar is currently in the year 2018 or 2019 depending on the time of year. Ethiopian official documents, newspapers, and signage routinely use the Ethiopian date, and tourists arriving in Addis Ababa quickly discover that 'the year' is not what they had assumed.
The Coptic calendar's role in Egypt is now mostly liturgical and agricultural. The Coptic Orthodox Church uses it for the entire liturgical year, and Egyptian farmers — Christian and Muslim alike — historically used it to schedule the agricultural cycle of the Nile, since the year is anchored to the heliacal rising of Sirius in early September, which closely approximated the timing of the ancient Nile flood. Egypt's civil state uses the Gregorian calendar, but the Coptic calendar continues in active religious and traditional use, and the Coptic months are still printed alongside the Gregorian months on most Egyptian wall calendars.
Modern proposals that didn't stick
The twentieth century produced several serious proposals to replace the Gregorian calendar with something cleaner, and all of them have failed. The pattern is consistent: a calendrical reformer notices that the Gregorian calendar is irregular (months of different lengths, the day of the week wandering against the date, leap years creating awkward jumps), produces a rational replacement, circulates it for political adoption, and runs into the same wall every time. The wall is the seven-day week.
The World Calendar, designed by the American Elisabeth Achelis in 1930, divides the year into four identical quarters of 91 days each, with each quarter consisting of one 31-day month and two 30-day months. The 364 days are supplemented by a single 'Worldsday' at the end of December that does not belong to any week (it is a date but not a day of the week), and a leap-year 'Leapyear Day' inserted similarly after June. Achelis's calendar gained serious political traction. The League of Nations debated it through the 1930s, the United Nations Economic and Social Council considered it in 1953 and 1954, and India and several other countries supported adoption. The proposal was killed by religious objection. The Jewish, Christian, and Islamic communities all objected that the 'Worldsday' broke the continuity of the seven-day week, and the US delegation, on the basis of religious freedom concerns expressed by American faith communities, effectively vetoed the proposal at the UN. The World Calendar Association still exists and the proposal is still circulating, but no government has adopted it.
The International Fixed Calendar, designed by Moses Cotsworth in 1902 and championed by George Eastman of Eastman Kodak through the 1920s, divides the year into thirteen months of exactly 28 days each. Twelve such months total 336 days, the thirteenth makes 364, and a single year-end day plus a leap day bring the count to 365 or 366. Every month begins on a Sunday and ends on a Saturday. Every date falls on the same day of the week every year. The thirteenth month, inserted between June and July, was to be called Sol. Eastman Kodak actually adopted the calendar internally in 1928 and used it for all internal bookkeeping until 1989, sixty years after Eastman's death. The proposal had serious traction internationally — the League of Nations preferred it to the World Calendar in some discussions — but ran into the same religious veto over the broken weekly cycle, plus a secondary objection that thirteen is a poor number for quarterly accounting (it does not divide evenly into halves or quarters).
The Hanke-Henry Permanent Calendar, designed by the Johns Hopkins economists Steve Hanke and Richard Conn Henry and published in 2012, is the modern revival. It is a 364-day calendar of twelve months in a pattern of four quarters, each quarter consisting of two 30-day months and one 31-day month, totalling 91 days per quarter. The year is leap-corrected by inserting an entire seven-day intercalary week at the end of December approximately every five or six years, rather than a single intercalary day, which preserves the seven-day week throughout the year. This is the cleverest of the modern proposals because it never breaks the weekly cycle. It has nonetheless gained no political adoption, on the grounds that the inconvenience of a calendar change far outweighs the convenience of every meeting falling on the same day of the week every year, plus a secondary objection that an intercalary week disrupts annual planning in ways that single-day intercalations do not.
None of these proposals are obviously wrong. The Gregorian calendar is in fact irregular, the days of the week do in fact wander, and there is in fact no good reason for February to have 28 days or for the year to start in the middle of winter. The reason none of them has been adopted is not technical but institutional. Calendar reform requires nearly universal coordination, and the cost of dissenting is asymmetric — the holdouts get to keep their tradition while the adopters bear the burden of conversion — which makes the holdouts a stable equilibrium and the reform an unstable one. The Gregorian calendar is bad in the same sense that English spelling is bad: not because nobody has noticed but because the cost of fixing it exceeds the cost of leaving it alone.
What we'll likely never change
The seven-day week is the single most stable cultural artifact in human history. It has run continuously, without interruption, since at least the Babylonian astronomical week of the sixth century BCE — roughly 2,600 years of unbroken count. It survives the fall of empires, the conversion of religions, the replacement of calendars, the imposition of new political systems, and the explicit attempts of revolutionary governments to break it. The French Republic tried with the ten-day décade and lost. The Soviet Union tried with the five-day Nepreryvka and lost again with the six-day Shestidnevka. The Gregorian reform cut ten days from the civil calendar in 1582, but the seven-day week kept running — Thursday 4 October 1582 was followed by Friday 15 October 1582, and the four was succeeded by a five in the weekday count exactly as if no days had been skipped at all. Most calendar reforms cut days; none have successfully cut a weekday.
The reason for the stability is that the seven-day week is not a calendar but a cycle. It is decoupled from solar and lunar reckoning, anchored only in itself, and reinforced by three of the world's largest religions independently. Judaism forbids breaking the cycle by halakhic law: the Sabbath must fall every seventh day from the original Sabbath, and any reform that interrupts the count violates the law. Christianity inherits this and adds the Lord's Day every seven days from the Resurrection. Islam adds Jumu'ah, the Friday congregational prayer. Three of the world's largest religions all stake their weekly liturgical practice on the seven-day cycle, and any state reform that disrupts the cycle is simultaneously picking a fight with all three. Historically, that fight has been unwinnable. Even the Soviet Union, with the most aggressive anti-religious policies of any modern state, backed down.
There is also the practical argument: seven divides poorly into anything (which is partly why the rational reformers want to abolish it) but it divides exactly evenly into one feature of human life that matters more than mathematical convenience. The seven-day cycle gives every fortnight, every month, every year the same internal rhythm. The same day of the week recurs every seven days without exception. Workers know when their rest day is. Markets know when they open. Schools know when they close. The cycle is short enough to be felt by the body — most people can predict their own energy and mood across the cycle without consulting a calendar — and long enough to give meaningful variation. Five-day weeks are too short to provide structure; ten-day weeks are too long to provide rest. Seven is empirically about right.
The remaining structural reforms of the calendar will almost certainly happen at the edges: leap-second policies, time-zone DST policies, the long-running discussion of how to handle the Mars sol in spaceflight contexts. The core of the calendar — the Gregorian year, the twelve months, the seven-day week — will almost certainly survive intact through the twenty-first century and well beyond. Every previous prediction of imminent calendar reform has been wrong, and the cumulative weight of failed reforms is now a positive argument for the Gregorian system's longevity. We are not going to be the generation that finally replaces it. The next reformer will be wrong too.
If you want the technical tools to work across the historical calendars, the multi-calendar converter handles every major living and historical system in one interface. The Julian-Gregorian converter is the focused tool for the 1582 to 1923 conversion problem. The Hijri converter, Hebrew calendar, and Chinese calendar cover the three largest non-Gregorian living systems. For the broader history of why we measure time the way we do, the time zones primer is the companion piece on the modern half of the same story.
Further reading: the Wikipedia article on calendar reform is a thorough survey of every serious proposal made in the last four centuries. The Encyclopedia Britannica entry on the Gregorian calendar is the standard reference for the 1582 reform. Robert Poole's Time's Alteration: Calendar Reform in Early Modern England (UCL Press, 1998) is the definitive scholarly treatment of the 1752 English transition. E. G. Richards's Mapping Time: The Calendar and Its History (Oxford, 1998) remains the best one-volume technical history of every major calendar system in current and historical use. For the technical specification of the leap-year rule and its astronomical justification, the US Naval Observatory calendar reference is the standard public-domain source.