Assuming your [fieldname] is something that's already been entered...
Create code that sets the ValidationText property of your date field to be what you want, ie,
[txtDateField].ValidationText = "The date should be greater than" & [referencefieldname].
Depending on what your referencefieldname is, you could put it in the 'On Lost Focus' of your referencefieldname, or in the 'OnCurrent' event of your form, or wherever, so long as it's before the user enters the Date in your validated DateField.
