Lying awake at this hour, the night stretches out with no shape to it at all, and guessing how much of it is left rarely helps. A live count to 12:59 am replaces the guess with a number that moves. Whether 12:59 am is when the alarm goes off or simply when you have given up on sleeping, the figure here is exact.
Coming back to this page in an hour, tomorrow or next month shows exactly the same thing: however long is genuinely left until 12:59 am next comes round. There is no session to lose and nothing to configure again, because the number is worked out fresh each time from the clock on your device.
British speech has a well-worn habit of dropping the middle out of a time: half seven means half an hour after seven, never half an hour before it. Quarter past and quarter to work the same way. Written down, the same moment appears plainly as 12:59 am, or as 00:59 where no ambiguity is allowed.
Common questions
Is it affected by the clocks changing?
Only in the sense that your device is. When the change between GMT and BST happens at the end of March and October, the device adjusts and the countdown follows it without anything to do here.
What is 00:59 in the 12-hour clock?
That is 12:59 am, or one minutes to one at night if you are saying it aloud. From 13:00 onwards, subtracting twelve gives the pm figure — the same moment written three different ways.
Does the number update by itself?
Yes. It recalculates every second from your device's clock, so there is nothing to refresh or restart. Whatever is on screen is the current gap to the target time, not a figure worked out when the page first loaded.
What happens if 12:59 am has already gone for today?
The count simply carries on towards the next occurrence, which is tomorrow. That rollover happens on its own — the page does not need reloading and there is nothing to reset by hand.