Validation calculations can be specified for both fields if you want:
Self > StartDate
Self < EndDate
But you may also want to use a script controlled by a script trigger such as OnObjectValidate to use the same logic to provide a more user friendly response. (I tend to put both a validation expression and a validation script in place. The calculation will always catch the error where the script trigger might possibly be bypassed by just the wrong user action or if I miss a detail in my layout design.
Also make sure you field Tab order has StartDate entry before EndDate or just rely on a script trigger such as OnObjectValidate
Because all checks require StartDate to have a value before EndDate
great thanks guys, i new it was a simple fix. thanks for your help and all working perfectly now.