Originally Posted by focusv8

From DVLA;
When to apply

If your tax is due to run out at the end of the month, you can renew it from the fifth day of the month.
If your vehicle has been untaxed, because you’ve just bought it, the tax disc has run out or a SORN is in force, you can tax from the first day of the month in which you apply.
If you want the tax disc to start on the first day of the next calendar month, you cannot tax your vehicle more than two days in advance.

And I believe you can't tax a Sorned car online before the end of the month otherwise it starts the tax from the beginning of that month.
Needs to done at Post office.