Early flights come with a chain of times attached — the cab, the bag drop, the gate — and every one of them sits well before the first trains run. If 2:21 am is the moment the front door has to close, this page counts down to it precisely, so nobody has to keep working it out in a dark hallway.
Coming back to this page in an hour, tomorrow or next month shows exactly the same thing: however long is genuinely left until 2:21 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.
Writing a time as 2:21 am 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 02:21 is less conversational and considerably harder to misread, which is why schedules and rotas lean that way.
Common questions
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 2:21 am, 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.
What is 02:21 in the 12-hour clock?
That is 2:21 am, or twenty-one minutes past two 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.