Most auctions have the timer started once the start bid has been made, so the auctioneer would have to be online at the minute the starting offer is made. Elsewhise I feel like there is room for error just as there is now. As it is now many people seem to struggle to know the starting point of an auction, so I dont think most timers would even be correct.
It is pretty easy to check starting times of auctions with the current state.
If the auction duration is 24 hours or 2 days, people would just need to add the duration to the date to know when the auction ends. If people are unable to do that simple thing, I dont know if an online timer would help or cause even more confusion.