Schedule Ship Date cannot be changed.

Hi Team

I am creating an order on vision operations instance.
On the sales order line, schedule ship date and schedule arrival date are getting defaulted as 31-DEC-2012.
If I am trying to change the date, it goes back to same after save.

I have already checked the defaulting rules and work day calendar but no help.

Please let me know what I am missing in the set up.

Thank You
Regards
Mirza Tanzeel