Fix leap year and month boundary arithmetic for datetime +/- timespan;

 the problem only manifests itself when adding years/months and hours
 when the hours:minutes:seconds pushes over midnight.
Fix interpretation of times with explicit timezone when the timezone is
 in daylight savings time and is not the default timezone.
Allow interpretation of explicit timezone when it is specified as two words:
 <standard time> DST". For example, "MET DST" (Middle European Time Daylight
 Savings Time). This syntax is found in the zic package on Linux boxes at least.