COUNTIFS formula problem - return blank or less than

I have a large COUNTIFS formula to summarise totals from a set of raw data.

I have run into a problem trying to count date values.

Part of the formula looks like this

Formula:

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It is only counting values in Column O that are less than the value in cell Q38 (so far so good) but at the same time, it is failing to count any cells that have no value in Column O.

I expected "<" to include empty cells on the basis that their default value should be equivalent to 0!

How do I alter the formula to include empty cells as well as less than?

In the end, I repeated the COUNTIFS formula (i.e. = COUNTIFS(formula) + COUNTIFS(formula)) with the 2nd version altered to

Formula:

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It works but I was hoping there was a more straightforward / less resource intensive way of achieving this result.

First, you need to realize that if you reference an entire column, you'll get a count of every blank cell through $O$1048576.
But if you have other criteria, you might add a second COUNTIFS that tests the same other criteria plus a test if Col_O is blank.....Something like this:

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Is that something you can work with?