DateAndTime::Zones
Methods
I
Instance Public methods
in_time_zone(zone = ::Time.zone)

Returns the simultaneous time in Time.zone if a zone is given or if Time.zone_default is set. Otherwise, it returns the current time.

Time.zone = 'Hawaii'        # => 'Hawaii'
Time.utc(2000).in_time_zone # => Fri, 31 Dec 1999 14:00:00 HST -10:00
Date.new(2000).in_time_zone # => Sat, 01 Jan 2000 00:00:00 HST -10:00

This method is similar to Time#localtime, except that it uses Time.zone as the local zone instead of the operating system's time zone.

You can also pass in a TimeZone instance or string that identifies a TimeZone as an argument, and the conversion will be based on that zone instead of Time.zone.

Time.utc(2000).in_time_zone('Alaska') # => Fri, 31 Dec 1999 15:00:00 AKST -09:00
Date.new(2000).in_time_zone('Alaska') # => Sat, 01 Jan 2000 00:00:00 AKST -09:00

Source: show | on GitHub 

# File activesupport/lib/active_support/core_ext/date_and_time/zones.rb, line 18
def in_time_zone(zone = ::Time.zone)
  time_zone = ::Time.find_zone! zone
  time = acts_like?(:time) ? self : nil

  if time_zone
    time_with_zone(time, time_zone)
  else
    time || to_time
  end
end