doubt regarding pop up message reminder

  I have two fields as follows

Lease Effective From   in datetimepicker 8/5/2010

Lease Renewal Due On in datetimepicker 7/4/2011

  i want to remind the lease renewal is only three month.i want to give pop up message.

how to do.please help me.please given the code.

thanks,

Rgds,
Narasiman P