I created a Customer Invoice at demo5.8.tryton.org with Invoice Date back-dated to 15-01-2021 When posting this invoice, I get this error message:
To number the invoice “(49)”, you must set an accounting date after " 01/15/2021" because the sequence “Invoice 2021” has already been used for invoice “2”.
I tried setting Other Info → Accounting Date to today or tomorrow, with no succes… (Only the date in the error message changes.)
What would you expect instead ?
Being able to post the invoice, of course. 
From this error message I don’t know what I can do to solve this. Just guessing that the message should read »the next number from sequence “Invoice 2021” has already been used for invoice “2”« I still don’t know what I, as a user, could to to solve this and get the invoice posted.
You should use a date after the one shown so the numbering is consecutive in time.
First of all: I was able to reproduce this issue on a fresh database (I had to, dince the demo database has been reset in the meanwhile):
I already did, see original post.
If I understand you correctly, I would need to increase the “Accounting Date” day by day for until I get to correct one? Is there some way for the user to learn which date to use? (I was just able to set an accounting date weeks in the future, so finding the correct date might be tremulous.)
Just for me to understand: What did went wrong here in the first place? Did somebody set Accounting Date for some invoice while he should not?
Indeed we are improving the error message to give the last date. So you just need to set the date to the one listed on the error message.
It seems so.
Normally the users will posts invoices without a date so the today date is set and users only need to take care when setting a date on the past. I’ve never seen a real user setting an invoice date on the future.
I’ve just tried.
I post an invoice dated 26 feb on demo.tryton.org
I cannot post an invoice dated pn today.
Posting post-dated invoices should be made difficult imho.
At least in smaller organizations post-dating invoices is somewhat common: Since the customer asks you to have the invoice dated for the last year. (Maybe I’m missing something here.)
post-dated : a date in the future
anti-dated ; a date in the past
In France, dates and numbers must grow.
As far as the invoice is in a diference fiscalyear it will be no issue if you use diferente sequences for the fiscalyear.
As you posted an invoice in 26feb you won’t be able to post another invoice with a data before 26feb.
Just filled Issue 10088: Add warning when posting invoices on the future - Tryton issue tracker to improve this.