Set 2nd datepicker max on select change based on value.

I have 2 datepickers, one auto filled with today's date and another one showing the date the budget will be valid.
i can't find a way of modifying that second picker based on a value, being the values 0 for custom validity date and 15,30,45 for a fixed amount of days based on that value.
Ex: i need the dates to change automatically when i select 15,30,45, but to leave the picker open when i pick the select with value=0.