class DateTime

def in_time_zone(zone = ::Time.zone)

DateTime.new(2000).in_time_zone('Alaska') # => Fri, 31 Dec 1999 15:00:00 AKST -09:00

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

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

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

Returns the simultaneous time in Time.zone. 
def in_time_zone(zone = ::Time.zone)
  if zone
    ActiveSupport::TimeWithZone.new(utc? ? self : getutc, ::Time.find_zone!(zone))
  else
    self
  end
end