JR pass use period- 24 hr?

If I.purchase a 7 day pass and exchange it on one day is it immediately activated or is it at the first usage? If I use it on a 9am train on March 24 is the last time I can use it 9am March 31? 24 hr cycle? Or does it have to be before 9am on 31?

Calender days

When you exchange your voucher, you choose the start date of the pass. It can be on the same day, or later, up to one month after the exchange.

You had to see the real ticket pic first.
https://4.bp.blogspot.com/-rSN__WEVXQc/Vr4FqNjtQ5I/AAAAAAAAQm4/TreCVmI...
No 24hr steps.
24hr steps one is "Tokyo subway ticket(Not JR's)" and some only.

In yr case(for 7 days pass), basically pass expired 30's night "last" train.(depends on travel area)
Mainly in mid Tokyo and Osaka/Kyoto and some lines still operating 24:00's like this.
http://ekikara.jp/newdata/ekijikoku/1301041/up1_13104011.htm
But last 30's night if took over night train (as over 24:00 and arr next morning),
you can reach that train's last stop(or booked get off stn) and more far until ticket gate out on 31's.
(covered only "base fare" no covered "LtdExp fee" and more)
See below samples with Google map,HyperDia or.

ex1)
Himeji(7th day)dep23:35 - (by LtdExp Sunrise Seto/Izumo,need nobi-nobi seat booking) - (8th date)arr7:08Tokyo
Tokyo(8th date)dep7:15 - (by LtdExp N'EX/need LtdExp fee) - arr8:10NRT(T2/T1) for fly out.
You cannot ticket gate out at Tokyo stn as stop over.

ex2)
Himeji(7th day)dep23:35 - (by LtdExp Sunrise Seto/Izumo,need nobi-nobi seat booking) - (8th date)arr7:08Tokyo
Tokyo(8th date)dep8:20 - (by Shikansen Hayabusa/need LtdExp seat fee) - arr12:12ShinHakodateHokuto
ShinHakodateHokuto(8th date)dep12:34 - (by LtdExp Super Hokuto/need LtdExp seat fee) - arr15:30MinamiChitiose
MinamiChitiose(8th date)dep15:39 - (by Rapid) - arr15:42CTS for fly out.
You cannot ticket gate out at Tokyo,ShinHakodateHokuto and MinamiChitiose stns as stop over.

ex3)
Tokyo(7th day)dep22:00 - (by LtdExp Sunrise Seto/Izumo,need nobi-nobi seat booking) - (8th date)arr6:27Okayama
Okayama(8th date)dep7:05 - (by Shinkansen Hikari/need LtdExp fee) - arr9:36Hakata
Then take city subway(not JR) to FUK for fly out.
You cannot ticket gate out at Okayama stn as stop over.

(note)
Late Mar new timetables update yet.
2017 JR group new timetables starts fr first SAT 4 Mar.
https://en.wikipedia.org/wiki/Japan_Railways_Group#/media/File:JR_Rail...

The day you activate it for counts as Day One - which starts and ends at midnight - so if you activate it to use it immediately at 6 PM, your first "day" is just 6 hours long.
You can also exchange your order for the pass and designate a future date to start on it - so for example to start using your pass from the earliest train possible (sometimes departing before the office even opens) or to leave from another station where you can't exchange your order for the pass, you can set the first day as tomorrow, or 3 days from today, etc.

Great thanks -- so basically if I get a 7 day pass on March 25 at 8am I have until March 31 at 12am or whatever is the latest JR train from wherever I am.

Also my understanding is no need for JR pass in Tokyo correct?

JR pass within Tokyo can be useful for the JR trains, but doesn't cover all the other train operators and subway. But there are still some lengthy journeys around the city where it would be useful. Just don't expect it to be a great money saver for those trips.

Only for Tokyo?
Yes, so no need. (too much)
Use Google map and HyperDia for calc.
https://goo.gl/maps/XQk9iFieZ192
(No "JR" icon trains are not covered except Tokyo Mono Rail)
https://www.jreast.co.jp/e/img/renew/common/logo.gif

But, Where to go? How to get in Tokyo?
Are those places really in "Tokyo" as location address?