website event countdown timer with server sided event

i'm working on a website. I need to add a timer to the page which counts down from for example : DD::hh:mm 02:12:34 to 0. once the timer hits 0, a query has to be sent to the database to notify it that the timer has ended.

How would i be able to make this timer?

also, the query at the end of the timer count down has to be sent regardless of current open sessions. (so also with 0 active sessions the event still needs to be handled)

edit: i don't have a machine to run a timer program