Data Validation not responding as expected

I have a rather simple formula that I'm using to validate a date BUT the validation message is not being triggered when the value of the conditions are true.

I've tested the formula externally and it produced a 0 (FALSE) and 1 (TRUE) as expected when fed the proper data. 

The formula and data are:

Date_Paid: Short Date (Z205) (The date a purchase was paid for)

Date_Posted: Short Date (AA205) (The date that the purchase was posted by the bank)

Posting_Validation_Date: Short Date (AC205) (The latest date that the ledger was balanced to)

Todays_Date: Short Date: From "TODAY()" on another sheet as a "Workbook Wide Entry"

The formula is: =IF(OR(AA205<Z205,AA205>Todays_Date,AA205>AC205),TRUE,FALSE)

The basis for the test is: The Date_Paid (Z205) can be anywhere in the current month, which is tested elsewhere, but the Date_Posted (AA205) can not be less than the Date_Paid (Z205) or greater than Todays_Date, or less than or equal to the Posting_Validation_Date (AC205).

Anyone with an idea???

I've tried MS support and they sent me here. Really need to get this as occasional keying errors sometimes cause transactions to be back posted and really mess up the bookkeeping.

Thanks

TheOldPuterMan AKA John and The GrumpyOldMan!!