Convert string to datetime in specific timezone - ruby-on-rails

Question

I'm using datetimepicker and need to save the string datetime obtained from params to a datetime in a specific, user dependent time zone. It will allow me to save proper UTC datetime to database.
params[:notify_at] #=> "2014-07-05 14:30:00"
user.time_zone #=> #<ActiveSupport::TimeZone:0x00000007535ac8 #name="Warsaw", #utc_offset=nil, #tzinfo=#<TZInfo::TimezoneProxy: Europe/Warsaw>, #current_period=nil>
And I would like to do something like:
date = params[:notify_at].to_datetime(user.time_zone) #=> Sat, 05 Jul 2014 12:30:00 +0000
(its 14:30 in user's localtime but 12:30 in UTC)

Answer 1

You can use the in_time_zone method. For example:
DateTime.current.in_time_zone("Alaska")
# => Fri, 23 May 2014 07:21:30 AKDT -08:00
So for your use case:
params[:notify_at].to_datetime.in_time_zone(user.time_zone)
Pro Tip: If using Rails v4+ you can actually do this directly on the string:
"2014-07-05 14:30:00".in_time_zone("Alaska")
# => Sat, 05 Jul 2014 14:30:00 AKDT -08:00
UPDATE
You can parse a string directly into a time zone (where the String should already be IN that time zone) like this:
Time.zone.parse("2014-07-05 14:30:00")
# => Sat, 05 Jul 2014 14:30:00 CEST +02:00
So for your use case do:
user.time_zone.parse(params[:notify_at])

Answer 2

Try this:-
date = params[:notify_at].to_datetime.in_time_zone(user.time_zone)
Rails console output:-
1.9.3p385 :004 > d= "2014-07-05 14:30:00"
=> "2014-07-05 14:30:00"
1.9.3p385 :010 > d.to_datetime.in_time_zone("Pacific Time (US & Canada)")
=> Sat, 05 Jul 2014 07:30:00 PDT -07:00
1.9.3p385 :011 > d.to_datetime.in_time_zone("Alaska")
=> Sat, 05 Jul 2014 06:30:00 AKDT -08:00
1.9.3p385 :012 > to_datetime.in_time_zone