Rails Time.zone.parse bug

gistfile1.rb

1.9.3p125 :003 > Time.zone = "Pacific Time (US & Canada)"
 => "Pacific Time (US & Canada)" 
1.9.3p125 :004 > Time.zone
 => (GMT-08:00) Pacific Time (US & Canada) 
1.9.3p125 :005 > Time.zone.parse("2012-03-25 03:29")
 => Sun, 25 Mar 2012 04:29:00 PDT -07:00 
1.9.3p125 :006 > Time.zone.parse("2012-03-24 03:29")
 => Sat, 24 Mar 2012 03:29:00 PDT -07:00 
1.9.3p125 :007 > Time.zone = "UTC"
 => "UTC" 
1.9.3p125 :008 > Time.zone
 => (GMT+00:00) UTC 
1.9.3p125 :009 > Time.zone.parse("2012-03-25 03:29")
 => Sun, 25 Mar 2012 04:29:00 UTC +00:00 
1.9.3p125 :010 > Time.zone.parse("2012-03-24 03:29")
 => Sat, 24 Mar 2012 03:29:00 UTC +00:00 

Found the reason. Time.zone.parse uses by default Time.parse, which internally uses the system timezone to determine the time. Thus, regardless of the chosen Time.zone, it will base the jump to DST on the system clock. On the other hand ActiveSupport::TimeWithZone takes care of the same thing for the current Time.zone, which means every year we have two missing hours instead of only one.

I think this would be fixed by just comparing time.hour to date_parts[:hour] (if it exists), and if they don't match, fall back to DateTime.parse. This bug can't really happen when the hour isn't specified in the time string, so the fact that DateTime.parse doesn't take an optional now argument shouldn't matter.

I'll have to cook up a patch with tests for this.

I added an issue for this: rails/rails#5559

...and a pull request: rails/rails#5560