
  -spec erlang:localtime_to_universaltime(Localtime, IsDst) ->
                                             Universaltime
                                             when
                                                 Localtime ::
                                                     calendar:datetime(),
                                                 Universaltime ::
                                                     calendar:datetime(),
                                                 IsDst ::
                                                     true | false |
                                                     undefined.

  Converts local date and time to Universal Time Coordinated (UTC)
  as erlang:localtime_to_universaltime/1, but the caller decides
  if Daylight Saving Time is active.

  If IsDst == true, Localtime is during Daylight Saving Time, if 
  IsDst == false it is not. If IsDst == undefined, the underlying
  OS can guess, which is the same as calling 
  erlang:localtime_to_universaltime(Localtime).

  Examples:

    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
    {{1996,11,6},{12,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
    {{1996,11,6},{13,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
    {{1996,11,6},{13,45,17}}

  Failure: badarg if Localtime denotes an invalid date and time.
