date: string of format: 2011/10/15 05:00:00 or 2011/10/15-05:00:00 or 2011-10-15 05:00:00 or 2011-10-15-05:00:00 or 2011-10-15T05:00:00 or 2011-Oct-15 etc. The time portion is optional.
Returns:
integer day number, e.g. 288, unless fractional=True, in which case the fraction of the day is also applied