A bid must be RECEIVED and timestamped at eBay's servers before the end of the last second. How long before that end you must submit it so it is timely received depends on how much latency exists at that time in your clock, nervous system, finger muscles, device, connection to your ISP, connection from your ISP to eBay, etc.
Note that the countdown you see is NOT a real-time feed of the time remaining and is not reliably accurate for placing a bid in the last few seconds (it does not take latency into account, which it would need to both ways). I highly recommend use of a "sniping service" (do a websearch, there are reliable and secure one with free use for limited bidding or different fee schedules to match your bidding frequency and ratio of wins to losses). Not only is it more reliable than planning to snipe manually, it forces you to calculate your TRUE maximum well in advance and bid that, which is the ONLY winning strategy on an eBay auction. Trying to react to bids coming in, especially in the last minute, is the second worst strategy (bidding MORE than the MOST you are willing and able to pay is the worst).
It used to be fairly easy to synch a local clock to the time remaining display in a way that took into account latency in both directions, but the active countdown killed the static display that made that possible. If you want to try your local clock recently synched to an atomic clock, be aware that neither ebay.com nor its mobile apps directly display the end time of an active auction to the second. The Listing Page displays it to the minute, but hh:mm is not the same thing as hh:mm:00.000. eBay truncates (rounds down) its time displays to the lower second, minute, or even hour depending on the page you are looking at and in some cases how much time remains. hh:mm could be anywhere from hh:mm:00.000 to hh:mm:59.999.
For an active auction with a bid you can calculate the endtime to the second from the starting time and duration displayed on the Bid History Page IF there has been a bid (there is no direct display on that page, and if there has not been a bid the Starting Price line with the start time does not display; copy the item number and paste it into the URL: ebay.co.uk/itm/########## to get the endtime to the second in London time (BST in Summer, GMT in Winter).
But it's far easier and more reliable to simply use a snipe service (do a websearch, we're not allowed to make recommendations unsolicited); there are reliable and secure ones with limited free usage and different payment setups to match your needs.
