Thread by @bjhomer

Javascript and Swift apparently disagree about when January 1, 1 AD was. (Both numbers in milliseconds-since-1970)

JS:
-62135596800000

Swift:
-62135769600000

These two numbers are exactly 48 hours apart. Can anyone explain what’s going on here?

Okay, I’ve found *part* of the problem. When the Gregorian Calendar was first instituted, then needed to implement a 10-day jump to put the vernal equinox back in the right place. This meant that the day after October 4, 1582 was October 15, 1582.

Swift understands this. JS does not.

-12219296400 is the timestamp 1 hour before the beginning of October 15, 1582 (UTC). That means it should be October 4, 23:00 hours, because of the skipped days.

JS gives back October 14, 1582. A day that did not exist.

Okay, so we’ve found a difference. But… that’s a difference of TEN days. It still doesn’t explain why there are only 48 hours between the two “January 1, 1 AD”s.

Side note: Day One’s calendar is smart, and knows about these 10 missing days!

More exploration!

JS is off by:
9 days on Jan 1, 1500. (JS says Jan 10)
8 days on Jan 1, 1400
7 days on Jan 1, 1300
7 days on Jan 1, 1200
6 days on Jan 1, 1100
5 days on Jan 1, 1000
4 days on Jan 1, 900
4 days on Jan 1, 800
3 days on Jan 1, 700
2 days on Jan 1, 600
…

(…)
JS timestamps are off from the Gregorian calendar by:
1 day on Jan 1, 500
1 day on Jan 1, 400
0 days on Jan 1, 300 (!)
-1 day on Jan 1, 200
-2 days on Jan 1, 100

Now remember, there’s no year 0. The year before 1AD is 1BC.

-2 days on Jan 1, 1 AD
-2 days on Jan 1, 1 BC

Okay, so I think we’ve found the culprit. Leap years!

The Gregorian calendar specifies that leap years happen on every year divisible by 4, except not those divisible by 100 unless they are also divisible by 400.

The Julian calendar just did them every 4 years.

If I’m understanding this right, Javascript is using a *hybrid* calendar for its dates; using the Gregorian calendar for dates on or after October 15, 1582, and a Julian calendar for dates before that time.

Swift is using the Gregorian Calendar consistently throughout.

So, if you’re asking “how many seconds before Jan 1, 1970 was Jan 1, 1 AD”, you need to first answer “which calendar are you referencing?”

Follow-up here:

https://twitter.com/bjhomer/status/1499870738307694593

Turns out, this was in fact exactly backwards! Swift is the one using a hybrid calendar. That’s useful in general for presenting dates to a user, but probably less useful for constructing ISO8601 date strings. In this particular case, I think JS has it right!

https://twitter.com/bjhomer/status/1499832879202783233