How to use hayauri 21 ticket?

Is it okay if I buy Hayauri 21 ten or fifteen days before my departure? Or should I use it on the 21st day? I don't really understand how this ticket works. I'd appreciate your explanations, guys. :)

"Hayauri" means early sales.
There are various discount services of this name or similar names.

Regarding JR highway bus Hayauri discount:

This discount is to be applied to a one-way bus fare
for specific routes and buses, for a limited number of seats.

Is it okay if I buy Hayauri 21 ten or fifteen days before my departure?

That is too late.

When you choose Hayauri 21,
you should pay the amount at latest 21 days before your boarding date.

Note
: Depending on buses, you cannot change your ride with Hayauri discount.
: When you change your ride or cancel then remake your reservation,
a case may be where you cannot secure a new seat at a discount price.

- Japan Expressway Bus Net
https://www.kousokubus.net/JpnBus/en