I just spoke to the my family "Attorney" they explain official detail here what the term mean "On or About" is.
Let say you have a contract close on Jan. 31, 2017.
On: Mean they must close on that day of Jan. 31, 2017. About: Mean they can close anytime before Jan. 31, 2017. They can't close anything after that day unless they have to tell the reason or submit exception form for delay. It very simple and no need to discuss this back and forth.
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