depends on how you define a week exxactly but in general you can use datediff with either
w or
ww as the interval
datediff('?',{CON.OnSiteStart},currentdate())
here is the defintiions of w and ww from HELP
Use DateDiff with the "w" parameter to calculate the number of weeks between two dates. For example, if startDateTime is on a Tuesday, DateDiff counts the number of Tuesdays between startDateTime and endDateTime not including the initial Tuesday of startDateTime. Note however that it counts endDateTime if endDateTime is on a Tuesday.
DateDiff ("w", #10/19/1999#, #10/25/1999#)
Returns 0.
DateDiff ("w", #10/19/1999#, #10/26/1999#)
Returns 1.
Use DateDiff with the "ww" parameter to calculate the number of firstDayOfWeek's occurring between two dates. For the DateDiff function, the "ww" parameter is the only one that makes use of the firstDayOfWeek argument. It is ignored for all the other interval type parameters. For example, if firstDayOfWeek is crWednesday, it counts the number of Wednesday's between startDateTime and endDateTime. It does not count startDateTime even if startDateTime falls on a Wednesday, but it does count endDateTime if endDateTime falls on a Wednesday. For the examples, note that October 6, 13, 20 and 27 all fall on a Wednesday in 1999.
DateDiff ("ww", #10/5/1999#, #10/29/1999#, crWednesday)
Returns 4.
DateDiff ("ww", #10/6/1999#, #10/29/1999#, crWednesday)
Returns 3.
DateDiff ("ww", #10/5/1999#, #10/27/1999#, crWednesday)
Returns 4.