I have a multiple entry 90 days Schengen visa (category C) from 15 March 2014 till 21 Dec 2014.
With this visa I entered The Netherlands on 4 April 2014 and left there on 13 April 2014.
I am planning to re-enter The Netherlands on 19 Dec 2014 and leave there for London (I have a valid UK visa as well) on 21 Dec 2014, at 7 am from Amsterdam.
Am I permitted to stay in The Netherlands on 21 Dec 2014 or should I leave the country on 20 Dec evening itself?
Better safe than sorry, always when in doubt leave early when it comes to visas, to give yourself some buffer in case something comes up, otherwise you are good to leave on 21 December with no problems as your visa expires by the end of the day (23:59) on 21 December, 2014.
The 90 days includes the 21st so this is fine. As Gayot said, it expires at the end of the 21st.
I usually recommend leaving space in case of flight cancellations etc, but that's just normal risk management :-)
You have until the end of the day on the expiration date.
So if your expiration date is December 21, 2014, you can still be in the country until 23.59.59 on December 21, 2014.
Of course, you'll want to leave a bit before then, to make sure delays in flights or whatever don't cause you to accidentally overstay.
As a result, your 7am flight to Amsterdam should be just fine.
The Schengen visa expires at midnight, 23:59, on the expiration date. In your case this would be 21 Dec 2014.
The precise definition for this regulation is found at Regulation (EU) No 610/2013 of the European Parliament and of the Council and antecedents are given on that page as further links.
Once you have departed, you can use the Schengen visa calculator to determine if you have unspent leave in the Schengen zone.
