Implementation for Excel WeekNum() function.
Syntax: WeekNum (Serial_num,Return_type)
Returns a number that indicates where the week falls numerically within a year.
Serial_num is a date within the week. Dates should be entered by using the DATE function,
or as results of other formulas or functions. For example, use DATE(2008,5,23)
for the 23rd day of May, 2008. Problems can occur if dates are entered as text.
Return_type is a number that determines on which day the week begins. The default is 1.
1 Week begins on Sunday. Weekdays are numbered 1 through 7.
2 Week begins on Monday. Weekdays are numbered 1 through 7.