Can road tax be back-dated?

Im looking to buy a used car on the internet, and some of the classified ads say no road tax. If I buy such a car, do I need to pay overdue road tax, or just from the date I buy it?
Daz Song

You only need to pay tax from the date of acquisition. Its the responsibility of the previous owner to pay the cars road tax or declare the car SORN (statutory off-road notice) up to the point they hands you the V5 registration document.

However, the moment you become the new keeper, it becomes your responsibility. That means you must tax the car, even before driving it home.