Parse the iso date strings into millseconds from the unix epoch and use
that through the timetable logic instead of reparsing the strings over
and over.
If a span ended on midnight the next day header would be started on the
break and the sameDay check would match the day of the next span causing
the day header to be inserted into the break column. Fix by checking
that the last day header has a non-zero span in the same day check.
