just for fun, create a new section to the detail section, below the current section. Place your formula there. If it displays values, it is a causality thing...the formula is displayed before the value has come back from the subreport, then it is just a matter of figuring out how to get the subreport to run and the formula to display as desired.
If it is still blank, I'm not sure...you might try using a datetimevar instead of the datevar (i've never used this one, but my data fields are all datetime).
HTH