Calendar conversion: eras, leap months and week-years
In historical research or a project combining records from several countries, the numbers can be correct while the comparison is wrong. A day does not change when expressed in another calendar, but its fields and numbering can. Keeping that context matters more than copying an unexplained result.
Start with the calendar, not the writing language
A Gregorian date can be written in Arabic, and a date in another calendar can be written in English. Language does not identify the calendar; a calendar name may also need an era to make its year number unambiguous.
Keep a document’s month name and era when they are given. Month 1 is not necessarily January, and a year number alone does not identify where the count of years begins.
References: Adobe — Calendar systems, eras and month rules
A leap month is not an extra day
Some calendars insert a month in particular years. A Chinese date, for example, can need a flag distinguishing an ordinary numbered month from a leap month with the same number. Dropping that distinction changes the intended date.
A Gregorian leap year instead adds a day to February. Do not transfer a 12-month rule or one calendar’s month lengths to another; use the rules belonging to the date you are reading.
References: U.S. Naval Observatory — Introduction to calendars · Hong Kong Observatory — Gregorian–Lunar conversion table for 2025
Week-years and the limits of historical conversion
An ISO week date uses a week-year, a week number and a day within the week. Its weeks start on Monday, and early January can belong to the final week of the previous week-year.
For old documents, separate mathematical conversion from the calendar actually used in that place and period. Applying Gregorian rules to an early date does not prove that its author used them. A converter’s range rejection also does not establish that a historical date never existed.
References: Python — ISO calendar year, week and weekday · U.S. Naval Observatory — Introduction to calendars
Example: a weekly report crossing New Year
The Gregorian date 2021-01-01 is 2020-W53-5 in ISO week notation: week-year 2020, week 53 and day 5, Friday. The 2020 in that notation is not a mistaken Gregorian year.
When grouping a report by week, keep the week-year beside its week number. “Week 53” alone, or a week number paired with a Gregorian year from another field, can put a record in the wrong group.