Maybe I'm just crazy... So, with SPD I can add any time unit to a date and put that into a variable, but I can't seem to find anything that will let me calculate the difference between two dates. "Do Calculation" only lets me pick from numeric values. Am I missing something here? Seems like something which should be obvious.
Date comparison is not available "out of the box", but there are some workarounds.
Maybe this article: How to perform date calculations and construct dynamic strings in a SharePoint Designer workflow provides the answer?
Some more related information here: Modifying Date Formats in SharePoint Designer Workflows