I am currently working with a set of data that contains the Date of Birth (dob) of an observation and the # of Weeks of Gestation at delivery. I am trying to figure out a way to generate a variable which represents the Date of Inception by subtracting the number of weeks of gestation from the dob. So far I've used these commands, but the DoI variable I'm generating is giving me incorrect values which are probably due to week representing the week in year 1960.

*wrong
format dob %tw
gen weekob = wofd(dob)
gen doincep = weekob - gestwk

Any help would be appreciated as I am fairly new to Stata and this forum. Thank you so much for any time and effort!!