class String

def in_time_zone(zone = ::Time.zone)

is set, otherwise converts String to a Time via String#to_time
Converts String 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).parse(self)
  else
    to_time
  end
end