Defining date rule functionality in date rule ?

Hi Team,

I am new of XML coding for defining date rule, my requirement is if user enter date suppose today date is less than previous date it is not allowed.for this how to define date rule and XML doing.please help.

First time it is empty, after that user enter any date is less than today date should not allow..for this how to define date rule.

date rule type - SRV_CUST_BEG.

Thanks & Regards

Kalpana