Is it possible to add a months to the specific date?

Our requirement is to add either weeks, months, or years to the Contract start date.

As per the current unifier design, we have the formula option to add days only to the Date Picker field.

Based on the above requirement I need to convert either months or year to days but the client is not agreed to the fixed 30 or 365 days for either month or year respectively.

So anyone suggests to us How I can calculate the days based on the Month or year selected by the user to calculate the Contractor End date.