[ruby-dev:51186] [Ruby master Bug#18946] Time#to_date returns incorrect date

[duerst <noreply@ruby-lang.org>]

Issue #18946 has been updated by duerst (Martin Dürst).


This may be due to the Gregorian calendar reform (see https://en.wikipedia.org/wiki/Gregorian_calendar#Gregorian_reform, which says: "Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October").

Time seems to use a proleptic Gregorian calendar (the Gregorian calendar extended to before the Gregorian reform, see https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar) while Date seems to be using the Julian calendar before the 'official' reform date. Please note that the reform didn't get applied at that date everywhere; different parts of the globe changed at different times, and some churches still use the old calendar.

```
Time.local(1582,10,14).to_date
=> #<Date: 1582-10-04 ((2299160j,0s,0n),+0s,2299161j)>
Time.local(1582,10,15).to_date
=> #<Date: 1582-10-15 ((2299161j,0s,0n),+0s,2299161j)>
```

Also, please compare:
```
Time.local(1500,3,1).to_date
=> #<Date: 1500-02-20 ((2268983j,0s,0n),+0s,2299161j)>
Time.local(1500,2,29).to_date
=> #<Date: 1500-02-20 ((2268983j,0s,0n),+0s,2299161j)>
```
(in Time, there's no February 29 in 1500, because it's proleptic)
with
```
Time.local(1500,3,9).to_date
=> #<Date: 1500-02-28 ((2268991j,0s,0n),+0s,2299161j)>
Time.local(1500,3,10).to_date
=> #<Date: 1500-02-29 ((2268992j,0s,0n),+0s,2299161j)>
Time.local(1500,3,11).to_date
=> #<Date: 1500-03-01 ((2268993j,0s,0n),+0s,2299161j)>
```
In Date, there is a February 29 in 1500, because it's Julian.

See also `ri Date`. It may be a good idea to add a note to the documentation for Time, to say that it uses the proleptic Gregorian calendar, and this is different from Date.

Please note that dates are *relatively* easy if the date is close to the present, but the farther you move out, the more complicated it may get. 

----------------------------------------
Bug #18946: Time#to_date returns incorrect date
https://bugs.ruby-lang.org/issues/18946#change-98504

* Author: kei-p (Keisuke Ishizawa)
* Status: Open
* Priority: Normal
* ruby -v: 3.1.2
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Time#to_date returns incorrect date.

Actual Behavior:
```
Time.local(1499, 12, 27).to_date
=> #<Date: 1499-12-18 ((2268919j,0s,0n),+0s,2299161j)>
```

Expected Behavior:
```
Time.local(1499, 12, 27).to_date
=> #<Date: 1499-12-27 ...>
```



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