If text is not a number, the Value function will return #VALUE!
Look here for detailed description:
http://www.techonthenet.com/excel/formulas/value.php


I couldn't reproduce the #VALUE! scenario that you described, but maybe some
of those cells are formatted as text? Try to convert (select right-click
format cells date) them all to values and see what happens.

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Ryan---
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"Sal" wrote:

=IF(COUNT(K13:L13)<2,"",NETWORKDAYS(K13,L13)-1)

This formula already excludes weekends from the calculation.

Do you know how I could improve this formula so that it does not include
holidays in the calculation, I can specify the holidays, and I will not get
an error message if Column K is blank?

Or

Do you know how I can improve this formula so that if Column K is blank I
will not get a #VALUE! message where the calculation should be?

=NETWORKDAYS(K13,L14,Holidays!A1:A29)-1