Request Position Change - validation that the effective date of change,

Summary: In the request for position change screen, by default the effective date of change will be defaulted to system date. The requirement is to validate that this date should be equal to the original creation date of the position

Content (required): In the request for position change screen, by default the effective date of change will be defaulted to system date. The requirement is to validate that this date should be equal to the original creation date of the position. This validation is working fine in request position change screen. But the same validation is firing in new position creation screen as well which is not the right place. Request you to help me how to stop the validation from firing in New Postion Screen.