I have this formula:

=IF(ISBLANK([@[Declaration Date]]),"",[@[Declaration Date]]+IF([@[Declaration Date]]<DATE(2024,8,15),365,457))

It reads a declaration date, and then returns an application due date either 365 or 457 days out from declaration date, depending on whether the declaration date was before or after 8/15/2024.

I would prefer to have it add 12 or 15 months to the Declaration Date rather than 365 or 549 days but don't know how to do that. And I don't really know how Excel knows that the 365 and 549 in the formula indicates days instead of months, or years.

Thanks in advance...