class Date

Date.new(2000).in_time_zone('Alaska') # => Sat, 01 Jan 2000 00:00:00 AKST -09:00

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

Date.new(2000).in_time_zone # => Sat, 01 Jan 2000 00:00:00 HST -10:00
Time.zone = 'Hawaii' # => 'Hawaii'

is set, otherwise converts Date to a Time via Date#to_time
Converts Date to a TimeWithZone in the current zone if Time.zone or Time.zone_default

def in_time_zone(zone = ::Time.zone)
  if zone
    ::Time.find_zone!(zone).local(year, month, day)
  else
    to_time
  end
end