Using Negative Dates

I need a formula to either display yes or no depending on whether a project with many different steps is on schedule to be completed on time. Each step has an individual due date, so essentially I need a formula that says whether or not today's date is before or after all of those dates. The other issue is that sometimes, there are blank cells for the due date column, and I'm having trouble using the ISBLANK( function.

Here's what I have right now:
=IF(OR(ISBLANK(C5:C20),C5:C20-TODAY()>0), "YES", "NO!")
but it isn't working properly.

Can anyone help?? Thanks!