Why has entering a date changed in 4.7.16? Can it be reverted?
There was an issue due to some refactoring around the handling of date fields to try to standardise them. This lead to a problem most notably in birth date fields in 4.7.16 but i'm guessing can be found elsewhere. This issue was raised CRM-19982 

I suspect the same fix will most likely work for you. If you have a test site or would like to give a test a shot. I would recommend you look at the changes made to the files in the DAO directories here in the Pull Request and make the same changes on your server. 

That pull has been merged into 4.7.17 which will be released i think in the next week or so