Throw a splashed potion above your head, can be any potion. See if the duration you get from it starts off exactly as what the potion says the duration it should be, and you will see that it is one second off.
Like for example if you were to splash yourself with a splashed swiftness potions of 8 seconds let's say, and if you were to splash as many above your head as you can, you will see that it will start you off with 7:59 as the duration rather than 8:00 and then decrement afterwards.
Same concept with regular potions, and lingering potions, just that it's easier to spot and reproduce with splash potions.
It's rounding down the timer to the nearest second. Notice that it stays on 0:00 for a complete second at the end.