If all is well you should be able to convert the calculated formula into a Nintex formula using inline functions. If however, these functions prove insufficient, you could consider using a helper list where you have your calculated column for the weekno and a date. In the workflow you create a new item in that list with your date, store its ID, Pause for 1 minute (to allow the item to be created), query list/list lookup the calculated column value with the weekno and finally delete the item.