Time#to_datetime incorrect for times before the date of calendar reform

[jeremyevans]

Environment

jruby 9.1.13.0 (2.3.3) 2017-09-06 8e1c115 OpenJDK 64-Bit Server VM 25.144-b01 on 1.8.0_144-b01 +jit [OpenBSD-x86_64]
OpenBSD systemname.local 6.2 GENERIC.MP#140 amd64

Also tested on:

jruby 9.1.12.0 (2.3.3) 2017-06-15 33c6439 Java HotSpot(TM) 64-Bit Server VM 25.131-b11 on 1.8.0_131-b11 +jit [mswin32-x86_64]
Windows 10 Professional

Expected Behavior

Same Results as CRuby for the following code:

require 'date'
# after date of calendar reform in 1582: correct
p Time.at(-12000000000).utc
p Time.at(-12000000000).utc.to_datetime 
# before date of calendar reform in 1582: incorrect
p Time.at(-12500000000).utc
p Time.at(-12500000000).utc.to_datetime 

CRuby Results:

1589-09-26 02:40:00 UTC
#<DateTime: 1589-09-26T02:40:00+00:00 ((2301699j,9600s,0n),+0s,2299161j)>
1573-11-22 01:46:40 UTC
#<DateTime: 1573-11-22T01:46:40+00:00 ((2295922j,6400s,0n),+0s,2299161j)>

Actual Behavior

JRuby Results:

1589-09-26 02:40:00 UTC
#<DateTime: 1589-09-26T02:40:00+00:00 ((2301699j,9600s,0n),+0s,2299161j)>
1573-11-22 01:46:40 UTC
#<DateTime: 1573-11-12T01:46:40+00:00 ((2295912j,6400s,0n),+0s,2299161j)>

Note the change from 1573-11-22 to 1573-11-12 in last line of output.

[headius]

I suspect this is because Joda Time, the time library we're using, defaults to ISO8601 time, while I believe UNIX time is Gregorian/Julian. I'm testing a patch that uses GJ, but I'm on the fence about fixing this.

[headius]

Hmm actually this logic is mostly contained in date.rb, which does already use the Gregorian/Julian chronology by default.

My patch to modify RubyTime in a similar way did not appear to help: https://gist.github.com/headius/6ee8683674d1c38a330b1a6c7bc7d5f6

[headius]

Ok interestingly enough, if I modify our date library to use ISO chronology, we match MRI.

[--dev] ~/projects/jruby $ jruby time_bug_4822.rb 
1589-09-26 02:40:00 UTC
ISOChronology[UTC]
#<DateTime: 1589-09-26T02:40:00+00:00 ((2301699j,9600s,0n),+0s,2299161j)>
1573-11-12 01:46:40 UTC
ISOChronology[UTC]
#<DateTime: 1573-11-22T01:46:40+00:00 ((2295912j,6400s,0n),+0s,2299161j)>

So I'm not sure who's right now.

[eregon]

Fixed by 8da4e66 on master, see #4810 for details.