[ruby-core:46317] [ruby-trunk - Feature #6720][Open] Discrepancy between DateTime#strftime('%s') and Time#strftime('%s') before 1582-10-15

Issue #6720 has been reported by phasis68 (Heesob Park).

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Feature #6720: Discrepancy between DateTime#strftime('%s') and Time#strftime('%s') before 1582-10-15
https://bugs.ruby-lang.org/issues/6720

Author: phasis68 (Heesob Park)
Status: Open
Priority: Normal
Assignee: 
Category: ext
Target version: 2.0.0


I found some discrepancy between DateTime#strftime('%s') and Time#strftime('%s') on date before 1582-10-15

Consider this

C:\>irb
irb(main):001:0> require 'date'
=> true
irb(main):002:0> Time.at(DateTime.new(0).strftime('%s').to_i)
=> -0001-12-30 09:00:00 +0900

irb(main):003:0> Time.at(DateTime.new(1000).strftime('%s').to_i)
=> 1000-01-06 09:00:00 +0900

irb(main):004:0> Time.at(DateTime.new(1500).strftime('%s').to_i)
=> 1500-01-10 09:00:00 +0900

irb(main):005:0> Time.at(DateTime.new(1582).strftime('%s').to_i)
=> 1582-01-11 09:00:00 +0900

irb(main):006:0> Time.at(DateTime.new(1582,10,4).strftime('%s').to_i)
=> 1582-10-14 09:00:00 +0900

irb(main):007:0> Time.at(DateTime.new(1582,10,5).strftime('%s').to_i)
ArgumentError: invalid date
        from (irb):44:in `new'
        from (irb):44
        from c:/usr/bin/irb.bat:19:in `<main>'
	
irb(main):008:0> Time.at(DateTime.new(1582,10,15).strftime('%s').to_i)
=> 1582-10-15 09:00:00 +0900

irb(main):050:0> Time.at(DateTime.new.strftime('%s').to_i)
=> -4713-11-24 09:00:00 +0900

irb(main):009:0> Time.at(DateTime.new(1583).strftime('%s').to_i)
=> 1583-01-01 09:00:00 +0900

irb(main):051:0> DateTime.new
=> #<DateTime: -4712-01-01T00:00:00+00:00 ((0j,0s,0n),+0s,2299161j)>

I understand the dates between 1582-10-05 and 1582-10-14 are invalid.
But I cannot understand why the discrepancy changes for each year.

Is the discrepancy inevitable and should not use DateTime#strftime('%s') before 1582-10-15?

I think DateTime#strftime('%s') should return same result with Time#strftime('%s') always.



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