In some formal contexts, it is common to refer to dates not only by the Gregorian calendar but also by a count of years starting from a certain event. For instance, American presidential proclamations are dated by the years of the independence of the United States; British formal documents are often counted by reference to the regnal year of the current monarch.
The starting point of such eras is usually not January 1. How does this affect the count of the years? Does a new year, for the purposes of this count, begin on the anniversary of the starting point (e.g., July 3, 2025 would be in the 249th year of American independence and July 4, 2025 in the 250th year? Or does the count coincide with the Gregorian calendar, so anything from January 1 to December 31, 2025 would be in the 249th year because 2025 contains the 249th anniversary of independence? The former would be more historically correct, but the latter avoids an awkward change of the year in the middle of a calendar month and is easier to convert.
Is the exact starting date so important? For example, the current Emperor of Japan is Naruhito. He ascended the throne in 2019 (not in January…), so 2025 is Reiwa 7.
The Gettysburg Address was Nov 19 1863, so clearly Lincoln was counting a non-calendar year starting in the middle of 1776 as his reference point, not Jan 1. But “four score, seven years, and 138 days ago” doesn’t roll off the tongue as well.
If you’re asking about alternative calendars, they have their own reckoning which (usually) doesn’t match Gregorian, so January 1 or whatever name used for the same day is just an arbitrary day. But they may be used alongside Gregorian or Julian.
For example, I would say that I am in my 89th year and will be until the day I actually reach 89. That day (should I live so long) will be the first day of my 90th year.
Most examples I’ve seen adhere to the starting day alternative.
Trump’s military parade was on the 250th anniversary of the founding of the U.S. Army. The 250th anniversary of the United States will be celebrated on July 4, 2026 (despite being 13 years too early: can anyone give the date of the first Congress off the top of their heads?). Nevertheless, all of 2026 will be celebrated as the 250th anniversary, just as 1961 was celebrated as the Civil War centennial.
I don’t remember any style guide advice on this, and I suspect it’s highly variable depending upon individual examples and context. Although some cultures designate age as years since the year of birth, western culture doesn’t roll over your birthday until the actual day. 9/11 and 12/7 will always be celebrated on those dates. OTOH, we say it’s been 116 years since William Howard Taft became president and 95 years since he died, with no one caring about the individual days.
Congratulations, Hari, you’ve long outlived your namesake!
One of the best contemporary criticisms I’ve read on Foundation was that it took place thousands of years in the future in a vastly advanced society yet Hari Seldon died of old age at 81.