How to use date for calculating values in other cells

[Bamboozled]

I want to include the date in the "If" statement for calculating values in
other cells.
Example: If the start date is after 1/1/1999, then A-B.

 

[dan dungan]

I want to include the date in the "If" statement for calculating values in
other cells.
Example:  If the start date is after 1/1/1999, then A-B.  

If the start date is in c1, would this work?

=if(c1>1/1/1999,a1-b1,"")

 

[Spiky]

I want to include the date in the "If" statement for calculating values in
other cells.
Example:  If the start date is after 1/1/1999, then A-B.  

A date in Excel is just a number formatted to look like a date. So if
you know the number, you can use greater than or less than just like
any other number. 1/1/1999 is actually 36161. Or you can put your
control date in another cell (B1 below) and just reference it.
Something like:
=IF(A1>36161,A2-B2)
=IF(A1>B1,A2-B2)