DateDiff in Access

I have a Field Named RTDDate and a Field that is FURDate and they are both set as Date/time in the properties. I then have a total field that is currently set as Number. I input the following code in the FURDate field after update CtlTotal = DateDiff(“D”, “RTDDate”, “FURDate”). I want Total field to show me the number of days between these two dates. I get a Runtime Error ’13’ type mismatch. HELP. I don’t care if the formula is in the FURDate after update or the Total field upon entry or what I just need to see the difference between the two dates.