21 Day JR Pass: Question on expiry date

This is probably a really stupid question, but I need to make sure. So, if I start my 21 day JR Pass on September 7th, when is the last day I can travel using it - is it the 28th, or the 27th?

I need to get back up to Tokyo on the 28th so it is very important! Thanks!

If you start on the 7th, the last day you can use the pass is 27th. It will not be valid on 28th.

If you take an overnight train on day 21 before midnight, it's still valid on day 22 as long as you don't clear the exit en route.

but what do you mean by 'clear the exit en route' thanks!

--what do you mean by 'clear the exit en route'--

You shouldn't get off the overnight train and leave the train station premises until the train get to your destination.

Quote from the JR Pass website:

If you travel overnight on the last day your pass is valid, the basic fare will be covered as far as you travel, including train changes, until you first exit a station. However, surcharges are only covered for the train on which you were riding past midnight on the final day your pass was valid.

This can be interpreted as follows:

e.g. If you have a 7-day pass, and take the 'Nobinobi seat', a carpeted semi private compartment in the limited express 'Sunrise Seto-Izumo' from Himeji at 23:35hrs on the last day of the pass to Tokyo, you don't need to pay anything for the following day's portion on that particular train ride. The train arrives at Tokyo at 7:08 next morning.

Furthermore, you can take connecting trains to, say, Sapporo on the 8th day of the pass as long as you don't get off the train and clear the turnstile en route. In that case, you will be imposed of the limited express surcharges between Tokyo and Sapporo. Only the base fare is valid on that segment, even though the 7 day JR pass superficially looks void on day 8. The Shinkansen - (limited) express surcharge connection discount scheme works in this case as well.

The case above in the second paragraph ( Tokyo to Sapporo trip on date 8 ) is idiosyncratic, therefore those who try to carry out a similar itinerary should carry the fine print of the JR pass in Japanese with you so that the ticket counter clerk and the conductors might not mistreat you by producing it to them.

I contacted JR East, and the rep thoroughly scrutinized this matter ( it took half an hour ).