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 8:59 am and this page will count down to it, exact to the second, for as long as it stays open.
The count makes no sound and asks for no attention while it runs. It is a number, not an alarm, and nothing goes off when it reaches zero. Leaving the page open in a background tab is perfectly fine — it will carry on counting towards 8:59 am whether or not anyone is watching it.
Said out loud, 8:59 am usually comes out as one minutes to nine 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
What happens if 8:59 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 8:59 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.