if I understand the question right, this is not possible in outlook as it
doesn't count weeks - you can use the method here:
http://www.outlook-tips.net/howto/countdown.htm to do a countdown.

But if you mean you need an appointment x weeks from mar 19, then open an
appointment form set to the 19th (either click on the 19th or enter 3/19 in
the date field and tab out) then select the date field and type xw where x
is the number of weeks - ie 3w for 3 weeks - then tab out and the date will
update (Apr 9 in this example)