[AccessD] Synchronizing Subform to Main form based on a date

I'm pulling my hair out on this one.

I have a simple main form with two unbound combos, one for a location
and the other for a date.  The RowSource for the location combo is a
table that contains a record for each location - pulling the PK in
column 1 and Name in column 2 from that table.  Column 1 is bound and
its width set to 0 so that only the name displays.  The RowSource for
the date combo is a summary query pulling from a transaction table with
column 1 equal to Int(MyDateFromTrxTable) and column 2 equal to
Format(MyDateFromTrxTable,"mm/dd/yyyy").  The query is grouped on column
1 so that only one row for each date appears.  Column 1 is bound and
width set to 0 so that only the formatted dates appear.

I'm having trouble getting a simple subform ([Location], Int([Date]),
[ItemDetail]) to synchronize with the main form.  Using the location and
date values, the only way I've been able to get it to work is to expose
the first column of the date combo by setting its width to something
other than zero.  Of course, I don't want the date serial number to be
displayed on the form - just the formatted date.  

Note that I have restricted both the bound column of the combo and the
corresponding value in the subform to just the integer portion of the
date.  Thus, I don't think this is a matter of trying to match a
"date-only" value against a "date-with-time" value.

I work with subforms like this all the time, though I don't think I've
ever attempted to use a date value as part of the link.  Is there some
esoteric quality of date values that I need to know about here?  Anybody
have a clue about why this won't work as advertised?

Thanks!

Don McGillivray