The document discusses time zones and date/time formatting in JavaScript. It explains that UTC and time zones are different concepts. ISO 8601 is the best date format standard to follow. JavaScript libraries like Moment Timezone can be used to convert dates to different time zones. The key challenges are getting the correct time zone offset on the backend and front-end, especially accounting for daylight saving time changes.

1. Time Zones
in JavaScript
hodak
monterail.com

2. https://en.wikipedia.org/?title=Time_zone#/media/File:World_Time_Zones_Map.png

3. Daylight Saving Time Explained
by CGP Grey
https://youtu.be/84aWtseb2-4

5. <select>

7. Time Zone != Offset

8. UTC - time standard
GMT - time zone
(since 1972)

9. Date formats

10. Best case scenario - ISO 8601
offset
2015-06-25T07:38:12+00:00
2015-06-25T07:38:12+0000
2015-06-25T07:38:12+04:30
2015-06-25T07:38:12-01:00
2015-06-25T07:38:12-04

11. 2015-06-25T07:38:12Z

12. 2015-06-25T07:38:12Z
Zulu time == UTC

13. Other formats

15. 2018-10-06T23:21:00+01:00
My system’s offset: +2

16. date = '2018-10-06T23:21:00+01:00'
$filter('date')(date, 'dd-MM-yyyy')
=> 07-10-2018
moment(date).format('DD-MM-YYYY')
=> 07-10-2018
new Date(date).toString()
=> Sun Oct 07 2018 00:21:00 GMT+0200 (CEST)

17. date = '2018-10-06T23:21:00'
$filter('date')(date, 'dd-MM-yyyy')
=> 06-10-2018
moment(date).format('DD-MM-YYYY')
=> 06-10-2018
new Date(date).toString()
=> Sun Oct 07 2018 01:21:00 GMT+0200 (CEST)

18. date = '2018-10-06'
$filter('date')(date, 'dd-MM-yyyy')
=> 06-10-2018
moment(date).format('DD-MM-YYYY')
=> 06-10-2018
new Date(date).toString()
=> Sat Oct 06 2018 02:00:00 GMT+0200 (CEST)

19. date = '2018-10-06'
$filter('date')(date, 'dd-MM-yyyy')
=> 06-10-2018
moment(date).format('DD-MM-YYYY')
=> 06-10-2018
new Date(date).toString()
=> Fri Oct 05 2018 21:00:00 GMT-0300 (BRT)
My system’s offset now: -3

20. WHAT IF
We’d like to show date and time in user’s time zone
that doesn’t equal her operating system’s time zone?

22. AngularJS

23. AngularJS

25. angular.module('project').filter('myDate', function($filter, User) {
return function(input) {
return $filter('date')(input, 'dd-MM-yyyy', User.current.timezone_offset);
};
});

26. How to get offset?

27. 1. It’s back-end problem
tz = ActiveSupport::TimeZone.new("Europe/Warsaw")
tz.utc_offset
=> 3600 # offset in seconds: +1 hour even though
currently it’s DST and it should be +2
tz.now.dst? # now -> passing the context
=> true
tz.at(6.months.ago).dst?
=> false
tz.now.utc_offset
=> 7200

28. 2. It’s front-end problem
jmp momenttimezone

29. Moment Timezone

31. moment('2018-10-06T23:21:00+01:00')
.tz('America/Los_Angeles')
.format()
=> 2018-10-06T15:21:00-07:00

32. moment.tz
.setDefault('America/Los_Angeles')
moment('2018-10-06T23:21:00+01:00')
.format()
=> 2018-10-06T15:21:00-07:00

33. Possible problem?
'America/Los_Angeles'
Rails match this format:
https://github.com/rails/rails/blob/a0a6e0292f66b546a3aee6893136a89bfd10579c/activesupport/lib/active_support/values/time_zone.rb#L43

34. Thank you
Questions?