Conditional format of field based on date compairson in crosstab

[BandOQ4b]

I have a cross-tab with date columns and store rows. The dates are dates of message transmission. The cells in the cross-tab is the number of messages sent from store to home office. This report deals with new stores for a range of days (variable). I need to show the number of messages when the store after the store opens - even if zero. However, I want the cell to be blank before it opens. The store open date is not in cross-tab, but in data in a table in the report. I tired format editor on the cell and putting a formula in "Display String" where Transmit_Date < Open_Date then ' else ToText (CurrentFieldValue, 0,') but it doesn't work.
Assistance greatly appreciated!!