# Whatever happened to the leap century? **URL:** \ **Category:** Factual Questions\ **Created:** [November 24, 2003, 1:56am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916 "2003-11-24T01:56:39Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![DirkGntly](https://avatars.discourse-cdn.com/v4/letter/d/d9b06d/32.png) [@DirkGntly](https://boards.straightdope.com/u/DirkGntly)\ **Post date:** [November 24, 2003, 1:56am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/1 "2003-11-24T01:56:39Z")
I understand that we observe leap year every four years to keep our calendars corrected for our actual solar orbit. I also understood that though we make this correction, an additional correction every 400 years was also necessary to keep things accurate. I recall that the year 2000 was supposed to be one of these “leap centuries” wherein February would actually have 30 days, and yet, no 30-day February in 2000. What happened? ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [November 24, 2003, 2:06am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/2 "2003-11-24T02:06:45Z")
It actually works the other way around; we need to drop a leap year day every so often. From [here](http://www.mitre.org/tech/y2k/docs/LEAP.html): > [@](#): > > If the year is divisible by 100, it’s not a leap year UNLESS it is also divisible by 400. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [November 24, 2003, 2:08am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/3 "2003-11-24T02:08:29Z")
**Dirk** , I’ve never heard of anything of the sort. 2000 was a leap year because it was evenly divisible by 400, which is the only time a century year can be a leap year.\* 1900 wasn’t and 2100 won’t be by the rules in place now. There’s no such thing as a leap century and February cannot have 30 days by the rules now in place. \*This made it somewhat easier for those who labored to make the Y2K bug fizzle. A naive algorithm for determining if a year is leap is to see if it’s evenly divisible by four. Since 2000 was divisible by both four and 400, overly-simple code correctly flagged it as a leap year. ---
**Author:** ![DirkGntly](https://avatars.discourse-cdn.com/v4/letter/d/d9b06d/32.png) [@DirkGntly](https://boards.straightdope.com/u/DirkGntly)\ **Post date:** [November 24, 2003, 2:09am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/4 "2003-11-24T02:09:15Z")
Ok, so what happened? We still had 29 days in Feb. that year, not 28… ---
**Author:** ![Random](https://avatars.discourse-cdn.com/v4/letter/r/57b2e6/32.png) [@Random](https://boards.straightdope.com/u/Random)\ **Post date:** [November 24, 2003, 2:36am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/5 "2003-11-24T02:36:07Z")
Read the responses again, **Dirk**. I can’t explain it any clearer than others already have. ---
**Author:** ![DirkGntly](https://avatars.discourse-cdn.com/v4/letter/d/d9b06d/32.png) [@DirkGntly](https://boards.straightdope.com/u/DirkGntly)\ **Post date:** [November 24, 2003, 2:45am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/6 "2003-11-24T02:45:18Z")
I just followed up on Ringo’s link…that pretty much summed it up. Thanks! ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [November 24, 2003, 2:46am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/7 "2003-11-24T02:46:16Z")
The rule is fairly simple: A year is a leap year if it is evenly divisible by four, unless it is evenly divisible by 100 but not by 400. Hence: 1. Every year that is divisible evenly by four is a leap year, unless 2. It is evenly divisible by 100, in which case it is a common year, except 3. Years that are evenly divisible by 400 remain leap years. There have only been two century leap years under the Gregorian calendar: 1600 and 2000, and 1600 did not apply in Britain or Russia, which remained under the Julian calendar until much later. Is there another formula to fine-tune the days-per-year figure even more closely? I seem to remember a note that 4000 AD would not be a leap year in order to bring things even more closely in tune with the 365.2422 days per year – but I won’t swear to that. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [November 24, 2003, 3:08am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/8 "2003-11-24T03:08:32Z")
> [@](#): > > … > Is there another formula to fine-tune the days-per-year figure even more closely? I seem to remember a note that 4000 AD would not be a leap year in order to bring things even more closely in tune with the 365.2422 days per year – but I won’t swear to that. IIRC, the former Soviet Union proposed an added adjustment involving the divisibility of millenial years to take care of the \< 1 day per millenium discrepency in the current rule, but the international standards bodies would not agree to it. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [November 24, 2003, 3:14am UTC](https://boards.straightdope.com/t/whatever-happened-to-the-leap-century/214916/9 "2003-11-24T03:14:00Z")
**yabob** : Of course they wouldn’t approve of it. In Soviet Russia, year leaps _you_. (I’m sorry.)