Bug #15988: Time#dst? vs "real" timezones

• Status: Closed
• Assignee: nobu (Nobuyoshi Nakada)
• Target version: -
• ruby -v: 2.6.3
• Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN

[zverok (Victor Shepelev)]

My current timezone is Europe/Kiev.

With default zone (how it was previously):

summer = Time.new(2019, 6, 1, 14, 30, 12)  # => 2019-06-01 14:30:12 +0300 
summer.zone                                # => "EEST" 
summer.dst?                                # => true 

winter = Time.new(2019, 12, 1, 14, 30, 12) # => 2019-12-01 14:30:12 +0200 
winter.zone                                # => "EET" 
winter.dst?                                # => false 

(Note it is correctly true in summer and false in winter.)

With "real" timezone object

require 'tzinfo'
z = TZInfo::Timezone.get('Europe/Kiev')
# => #<TZInfo::DataTimezone: Europe/Kiev> 

summer = Time.new(2019, 6, 1, 14, 30, 12, z)  # => 2019-06-01 14:30:12 +0300 
summer.zone                                   # => #<TZInfo::DataTimezone: Europe/Kiev> 
summer.dst?                                   # => true 

winter = Time.new(2019, 12, 1, 14, 30, 12, z) # => 2019-12-01 14:30:12 +0200 -- offset is correct
winter.zone                                   # => #<TZInfo::DataTimezone: Europe/Kiev>
winter.dst?                                   # => true -- this is wrong!

Note that offsets are calculated correctly (+3 in summer, +2 in winter), but dst? is always true.

TZInfo itself calculates DST properly:

z.dst?(summer) # => true 
z.dst?(winter) # => false 

Seems like a serious bug to me (or at least very hard to explain behavior).

PS: With arithmetics, everything works correctly:

six_months = 6 * 30 * 24 * 60 * 60
summer + six_months                           # => 2019-11-28 13:30:12 +0200
(summer + six_months).dst?                    # => false -- that is correct

[zverok (Victor Shepelev)]

Even funnier:

winter = Time.new(2019, 12, 1, 14, 30, 12, z) # => 2019-12-01 14:30:12 +0200 -- offset is correct
winter.dst?                                   # => true -- this is wrong!
(winter + 1).dst?                             # => false -- fixed!

summer = Time.new(2019, 6, 1, 14, 30, 12, z)  # => 2019-06-01 14:30:12 +0300 
summer.dst?                                   # => true 
(summer + 1).dst?                             # => false -- what?..

[jeremyevans0 (Jeremy Evans)]

• File time-zone-dst.patch time-zone-dst.patch added
• Status changed from Open to Assigned
• Assignee set to nobu (Nobuyoshi Nakada)

The behavior seems even worse in the master branch and 2.7.0-preview1, with a RuntimeError without a calculation, and always false after a calculation:

require 'tzinfo'
z = TZInfo::Timezone.get('Europe/Kiev')
t = Time.new(2019, 6, 1, 14, 30, 12, z)
t.dst?
# RuntimeError (isdst is not set yet)
(t + 1).dst?
# false
(t + 6 * 30 * 24 * 60 * 60).dst?
# false

Attached is a patch that calls dst? on the timezone object to determine whether the time is in daylight savings time in the zone. This does add a method called on the timezone argument, though, so I'm not sure how acceptable it is. Both the tzinfo and timezone libraries appear to implement the method already.

[nobu (Nobuyoshi Nakada)]

I've noticed it and have a patch, but haven't committed it yet as not written the test using dummy timezone objects.

[nobu (Nobuyoshi Nakada)]

• Status changed from Assigned to Closed

Applied in changeset git|149e414ed529d27aaeb0543bc133e08c782d8d41.

---

Initialize DST flag

• time.c (zone_timelocal): initialize DST flag by asking the
  timezone object. [Bug #15988]