OptionaldateOptionalshiftA boolean that indicates whether the created date's timezone
offset (as all Javascript dates are local) will be added to
the date instance. This has the effect of causing the resulting
date to "be in UTC" without such a thing really existing in JS.
Most importantly, this allows dates created with this method to
line up with server values (most good servers have their TZ set
to GMT) for operations like toUTCString().
A seed date, whose year-month-date values will be used.