Time is not added to an auction,* and the clocks that timestamp each bid coming in are very accurate and precise to a small fraction of a second. The DISPLAYS available to users, on the other hand, are terrible.
Your bid must be RECEIVED and timestamped at eBay's servers before the end of the last second. The countdown is NOT a real-time feed of bids coming in and eBay's clock counting down and is not an accurate measure of how much time you have left to submit the bid (click the confirm button) so it is received in time. It is rather a script that runs on your device periodically calling for updates on the status and time remaining, and uses your device's clock to count down from the time left display it last received from eBay (which took time to traverse the internet and get loaded into your device's RAM so your clock could start counting down). There is no adjustment for latency either way and so it is doubly slow compared to the time you would have to submit for it to get to eBay in time. And there are issues if your device's time settings are not right (wrong timezone, clock not well synched to a reliable time server).
Back before the countdown was introduced, it is was relatively easy (and fun) to synch a local clock to the time remaining by clicking refresh on the Bid History Page at a particular noted time per the local clock (top of a minute was easiest) and then adding the returned time remaining to that time; this automatically adjusted for latency both ways and gave you the second before which you had to click the confirm button (best to do that a few times and pick the earliest result and add a solid buffer time since latency can change). But the countdown replaced the static display that enabled you to get a single unchanging time left display upon refreshing so it's not so easy anymore. I now use and recommend a "snipe service" to place your one true maximum bid about 6 seconds out; mine allows limited snipes for free (up to 4 wins per month) or several different pay plans if you are using it more frequently (or want a bit more reliability that comes with a "mirror server" in another location than the main server) and uses an eBay supplied API instead of the glitch user interface. It also captures any error message that eBay returns if the bid is unsuccessful, as opposed to you using the user interface where any such error message is overwritten when the countdown gets to 0 so you generally don't even see it much less be able to read it. There are other snipe services out there with their own plans, but we are not allowed to make unsolicited recommendations of particular ones.
*On the Bid History Page of the full website (bid histories on the apps are worse than useless) compare the Starting Time (on the line for the Starting Price at the bottom) with your and the other bids to see the sequences and times. The auction ends exactly x days (the "Duration") to the second after the start (the displayed endtime at the top is truncated to the lower minute, the seconds will be that on the starting time).
