IF Statement

In Cell A2 I have a start date
The completion date in B2 is blank
In C2 I am simply calculating the days between B2 and A2, but because B2 is blank, C2 is showing a -42492 figure

How can I keep cell C2 blank " " whilst the completion date is unknown?

This is the formula I currently have that shows -42495

=IF(B2-A2=0," ",B2-A2)