Variable on 0fiscper3(posting period) with Manual entry as processing type

Hi gurus,

A variable ZIMFIS is on top of 0fiscper3 time characteristic. The variables processing type is manual/default entry. The requirement i have is, i need to create the variable as month range. But when i tried this , the values are coming from 1-54. Since it is month range i want the max value to be 12. It doesnt throw an error when i try to give 13, 14 etc. How can i handle this, that it shouldnt accept 13,14 etc( remember the variable is manual entry).

Regards,

Raj.