Hello Sean
When saving your variant,
1 - On the row of your date field, select the F4 help of the field 'selection variable' and choose the value D: Dynamic date calculation
2 - Using the F4 help of the field 'name of variable', choose the option 'Current date +/- ??? days'.
3 - A popup screen will appear to input the number of days.
BR
Caetano