Deciding to be asleep by a set time and actually managing it are two different problems, but the first is easier with a real number attached. This page counts down to 10:02 pm precisely, which makes it harder to tell yourself there is more of the night left than there really is.
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.
Writing a time as 10:02 pm keeps the am or pm attached, which reads naturally in a sentence but is easy to skim past in a long list of similar entries. Writing it as 22:02 is less conversational and considerably harder to misread, which is why schedules and rotas lean that way.
Common questions
What happens if 10:02 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 10:02 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.