Glad it works (so far)
Datediff calculates the difference in days between 2 dates, so yes, dividing by 14 should give you a fortnightly change.
the easiest way to test, (rather than waiting a week) is to change the arbitrary date in the WHERE clause.
2010-01-01 is the first of January, so there are about 20 or 21 weeks since this date.
Changing the date to 2010-01-08, or 2010-01-15 will give 1 or 2 weeks fewer than this, and should therefore return a different row.
I tried it with 5 rows and 5 different dates, 1 week apart and it seems to work ok