in_time_zone

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

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

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.

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