ALERT MESSAGE ON OLD MONTH SELECTION IN CLIENT PC Options

[shankar from vizag]

Greetings

I have two selection drop down boxes. One is for Month and the other one is for Year.

If, user selects pevious month, I need to display an alert box stating that "the selected month is older than ___ monhts".

Kindly guide me how to do javascript function onchange event.

I am attaching the php page.

Thank you .