Tea time in a house with children is less a meal than a schedule, with everything before it timed to land in the right order. If 3:59 pm is when it needs to be on the table, the countdown here is more use than an estimate, updating continuously as the afternoon runs down.
Because the count reads straight from your device's clock, its accuracy rests on that clock being right, time zone included. Phones and computers normally handle this themselves, so in practice the figure here lines up with whatever time the device is already showing elsewhere on screen.
Plenty of phones, laptops and dashboards default to a 24-hour display regardless of how people around here actually talk, so 15:59 may well be the figure staring back at you even though 3:59 pm is how the same moment would be said aloud. Neither is more correct than the other.
Common questions
What happens if 3:59 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 3:59 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.