Reserving a Seat before JR Pass Activate Date

It might be a dumb question, but it's better to be sure.
I'll be using the JR Pass on my travels around Japan next week.

My JR Pass will be activated on December 3. But before that date (December 3), when I already have exchanged my voucher and I am already in possession of the actual JR Pass ticket. Even before the activation date, can I already reserve a seat via the Shinkansen ticket machine on December 2? Even before the activation date of the ticket?

I'm just afraid that booking a reserved seat before my ticket's activation date will overwrite and suddenly activate my 7 days just by reserving.

I assume you refer to the Japan Rail Pass.

Yes, you can use machines and counters to make reservations already.
If you had used the official website to purchase the pass, you could also use the website. But it seems you used an unofficial sales channel, because you mentioned a voucher.

Official channels use vouchers. That has always been the case for many years that officaial agents provide an exchange voucher. The web purchase is a very late addition to the rail pass system.
Once you exchange your voucher, you can make reservations, and exchanging up to one month in advance of use has been the norm for many years.

With "official" I meant the official website:
https://japanrailpass.net/en/

@Uji - I see that "official" bandied about on the TA forum a lot, but it is really just another sales channel and hasn't actually been around that long.
I looked at signing up as a reseller - more for my own business after I was approached by one of the large travel companies - but the value proposition (margin/benefit) is tiny. Many of the 2020 changes have been positive and with the price increase and online booking the landscape for organising long distance trains has changed a lot in the past few years.

With the (nationwide) JR Pass, the advantage of being able to book online in advance (even before you get to Japan) and make changes as often as you want online (up until you actually collect tickets) is huge, so arguing about whether a source is gofficialh is not very meaningful. If you donft buy the pass through the JR site you have to wait until you get to Japan and exchange your voucher, and you lose a large amount of flexibility in terms of booking on the fly and being able to change your plans as often as you want without having to stand in line at a ticket office or machine.

Online purchase through JR is a game-changer, as far as Ifm concerned, and not just an alternative sales channel. I still buy nationwide JR passes for most of my trips, and for me they can still be quite cost-effective and also gfun-effective.h (I go places and do things I would never do if I didnft have a pass, and I also save a lot of money on hotels by using the pass strategically.) But Ifll always get them from the JR site that allows me to book online.