class Date

def xmlschema

date.xmlschema # => "2015-05-23T00:00:00+04:00"
date = Date.new(2015, 05, 23) # => Sat, 23 May 2015

defined by XML Schema:
Returns a string which represents the time in used time zone as DateTime 
def xmlschema
  in_time_zone.xmlschema
end