I've always thought since the VED changed so the 'new' owner has to purchase it as well (ie it does not 'transfer') apart from the 'extra' revenue HMG potentially get for a double month, the refund to the old owner is only complete months. It should not its electronic be calculated to that day, as the previous "unused months" made sense when you had a physical disc and postage to think about, but not now...
EDIT... I see their reply on that link:
A spokesman added: "Now all new keepers must tax the vehicle before they use it rather than having to find out if it’s taxed or whether the seller has had the tax refunded."
Well it was quite easy before as it either had a disc in the windscreen or it did not

, simples.

(Yes I know that is no guarantee but its a long way towards it)
I've always tried to buy cars with tax when I can, either privately or if from a dealer I ask them to tax it, this was not a ploy it was simply to save time, ie when I arrive at the sellers I swap the insurance over and as its already taxed and tested I'm ready to go. I guess it just means another phone call for some more open wallet surgery ie to pay for the 'new' tax as well. I think the next change I'll do at the start of the month ie buy on the 1st or 2nd etc this way neither myself or seller lose too much.
Andy.
91 205D-Turbo, gone but still missed
02 106D, TUD5B, gone but not really missed apart from the MPG