Hi,
If you have not find any Answer till now then try this.
Write your code in EXIT: EXIT_SAPLCATS_012
Program: SAPLCATS
Include: LCATSF8U
This exit has SAP_CATSFIELDS-INPUTDATE as Exporting parameter which will have the value that you are passing from CAT2 Tcode .
Here you can validate the periods and show the error message.
Hope it helps.
Thanks.