Generally speaking, a visa valid until the 17th means that you have to leave before midnight that day.
- If the reasons why you have to re-schedule are totally out of your control, you might be able to get an extension on humanitarian grounds. Examples might be an airport that closes down due to bad weather, or you or a close relative being in hospital with an illness that prevents travel. Contact the immigration authorities now, before the visa expires.
- Other reasons like saving a lot of money by flying later are not acceptable.
- If you were to get a new Schengen visa valid from the 18th, you would not have to leave between the midnight of the 17th and the morning of the 18th. But that will be difficult if the 17th is the 17th September.
Overstaying may result in a fine, and it will make future Schengen visa much more difficult.