Errands at this time of day get squeezed between other things, and the useful question is not what time it is but how much of the middle of the day is genuinely left. This page answers that against 12:23 pm exactly, updating on its own for as long as it stays open on the screen.
The figure on this page comes from your own device's clock, checked continuously and recalculated every second against 12:23 pm. There is nothing to press and nothing to set up first — the count is already running by the time the page has finished loading, and it keeps going while you are looking at it.
Plenty of phones, laptops and dashboards default to a 24-hour display regardless of how people around here actually talk, so 12:23 may well be the figure staring back at you even though 12:23 pm is how the same moment would be said aloud. Neither is more correct than the other.
Common questions
What happens if 12:23 pm 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.
Which clock does it use?
Your own. It reads the local time and time zone from the device you are looking at, so the figure reflects where you actually are rather than a fixed zone written into the page.
Will this alert me when the time arrives?
No. It is a live number rather than an alarm — nothing rings, buzzes or notifies you at zero. If you need to be interrupted at 12:23 pm, set an alarm as well and treat this page as the running total.
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.