Mojo::Date: epoch can be a negative number

[aferreira]

Summary

Fixes Mojo::Date when used with negative numbers as epoch

Motivation

I found the issue when representing birth dates with Mango::Time objects - a date before the epoch like 1965-12-21 would emit uninit warnings and end up represented as Jan 1, 1970. This comes from https://github.com/oliwer/mango/blob/master/lib/Mango/BSON/Time.pm#L12 which in turn points to Mojo::Date code https://github.com/kraih/mojo/blob/master/lib/Mojo/Date.pm#L25

For example, running the test included in this PR with the code in master gives the output

$ prove -I lib t/mojo/date.t
t/mojo/date.t .. 1/? Use of uninitialized value in gmtime at /Users/ferreira/ws/mojo/lib/Mojo/Date.pm line 66.

#   Failed test 'right format'
#   at t/mojo/date.t line 81.
#          got: 'Thu, 01 Jan 1970 00:00:00 GMT'
#     expected: 'Tue, 21 Dec 1965 00:00:00 GMT'
# Looks like you failed 1 test of 40.
t/mojo/date.t .. Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/40 subtests 

Test Summary Report
-------------------
t/mojo/date.t (Wstat: 256 Tests: 40 Failed: 1)
  Failed test:  33
  Non-zero exit status: 1
Files=1, Tests=40,  0 wallclock secs ( 0.02 usr  0.01 sys +  0.02 cusr  0.00 csys =  0.05 CPU)
Result: FAIL

References

None

[marcusramberg]

Seems like a good fix with a reasonable test.

[jhthorsen]

I didn't know negative epoch was a thing, but the actual fix seems reasonable to me as well.

[kraih]

Thank you, looks like this passed the vote.

[kraih]

Afraid i had to revert this change again, since once i added the missing tests for fractional negative epoch values it wasn't portable anymore. d87581f

[kraih]

Failing test can be seen here. https://travis-ci.org/kraih/mojo/jobs/218440126

[kraih]

GitHub won't let me reopen this pull request, please open a new one if you find a fix.