Assuming column A if one columns, B is column that can be blank, and C is
your constant:

=IF(B2="",ABS(A2-C2),ABS(B2-C2))

The absolute function is included to cover both scenarios in which constant
could be less than or greater than date in A2/B2. Feel free to take it out if
you don't need it.
--
Best Regards,

Luke M
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"dhealy" wrote:

I have 3 columns with dates in them with one column being the constant. I
need to find the number of days between the constant date and one column if
the other column is blank, if there is a date in the 3rd column then I need
to use that date with the constant. Help...