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So I think all you need to do here is employ the Show Missing Values (in your date pill ensure this is ticked)
Dates and Bins are known as "Range Aware" pills (in that Tableau knows their Min, Max and Increment) so they have this option. This creates Psudo-marks for the marks in-between the actual data, and so are used (even though there value is NULL) is any Table Calcs.
Hope that solves the issue, but let me know if not
It works. Thank you Simon.