If with currentDate

Hi,

I'm trying to create a dimention that will return the number of days that a certain contact is open.

This is the formula I wrote:

=If IsNull(CLOSING_DATE]) then CurrentDate() - (OPENING_DATE)

But an error message pops up:

'The expression or sub-expression at position 36 in the '-' function uses an invalid data type. (IES 10037)'