Appointments at the surgery or the dentist are rarely worth arriving late for, and the useful figure is not the appointment itself but how long is left before you have to set off. Set that in your head as 10:58 am and this page will count down to it, exact to the second, for as long as it stays open.
Nothing is downloaded, nothing is registered, and nothing is left on the device afterwards. The whole count runs inside the page you already have open. Closing it costs nothing at all, and opening it again later simply shows the accurate gap to 10:58 am as it stands at that moment.
Said out loud, 10:58 am usually comes out as two minutes to eleven in the morning rather than as a string of digits. Spoken and written time diverge more than people expect, which is worth remembering when somebody tells you a time over the phone and you are trying to match it against something printed.
Common questions
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 10:58 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.
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 10:58 am, set an alarm as well and treat this page as the running total.