java script not trigger more than once - on change event

I have one field in form to capture date.If we select less than 7 days from today's date within that ,it will show the error as per our coding.

If I select the first time it will work properly again i select the wrong date in the field it will not show the error.because the javascript wasn't trigger.

I trigger that function in date field's on change Event.

(for Ex) today's date is 15th april. I select 21st apr in that date field it shows error.again I select 19th apr the javascript was not triggered.

How can I solve this? Thanks in Advance .