Apple iWork '09 - Manual (Page 60)

WEEKNUM
The WEEKNUM function returns the number of the week within the year for a given date.
WEEKNUM(date, first-day)
 date: The date the function should use. date is a date/time value. The time portion
is ignored by this function.
 first-day: An optional value that specifies whether weeks should begin on Sunday
or Monday.
Sunday is 1 (1 or omitted): Sunday is the first day (day 1) of the week and Saturday
is day 7.
Monday is 1 (2): Monday is the first day (day 1) of the week and Sunday is day 7.
Example
=WEEKNUM(”7/12/2009”,1) returns 29.
=WEEKNUM(”7/12/2009”,2) returns 28.
Related Topics
For related functions and additional information, see:
“DAY” on page 47
“HOUR” on page 51
“MINUTE” on page 52
“MONTH” on page 53
“SECOND” on page 56
“YEAR” on page 62
“Listing of Date and Time Functions” on page 42
“Value Types” on page 36
“The Elements of Formulas” on page 15
“Using the Keyboard and Mouse to Create and Edit Formulas” on page 26
“Pasting from Examples in Help” on page 41
60 Chapter 3 Date and Time Functions