IN MY NOKIA E63 ( DEVICE SOFTWARE 500.09.21) WHEN I M INSTALLING ANY APPLICTION IT SHOWS CERTIFICATE EXPIRED. AFTERTHAT WHEN I CHANGE MY DATE SETTING TO EARLIER DATE THE ALSO IT SHOWS CERTIFICATE EXPIRE.. PLEASE SOLVE MY PROBLEM

Some 3rd party applications are signed with certficates which are valid only until specific date. Installing them after that date is normally not possible and results in an error message about expired certificate. You'd have to wait until the developer releases an updated .SIS file signed with new certificate.
However, there is a very simple workaround letting you install the software and use it without any restrictions. The only thing you have to do is to change the date of your mobile phone several months back, install the program and then switch the date back to the correct setting.