How to use the period in controls?

Hi,

I would like to use a control to validate my current opening balance amounts to the closing balance amounts from the final period in the prior year. However I cannot get it to work as I struggle to find what to enter in the period field for the control. I have tried DEC (PERIOD property in the time dimension) and 12 (MONTHNUM property in the time dimension) but in both cases BPC doesn't validate the control saying that neither value is valid.

Thanks,

Arnold