WeekStart( date [, shift = 0 [,weekoffset = 0]] )
Returns a value corresponding to a timestamp with the first millisecond
of the first date ( Monday ) of the calendar week containing
date. The default output format will be the DateFormat set in the
script. Shift is an integer, where the value 0 indicates the week
which contains date. Negative values in shift indicate preceding
weeks and positive values indicate succeeding weeks. If you want to
work with weeks not starting midnight between Sunday and Monday,
you may indicate an offset in days in weekoffset. This may be
given as a real number indicating days and/or fractions of a day
