Do you need a timer that will work when the program is closed?

Question

Java

A timer that will work even if the program is closed or the computer is turned off.
It is needed in order to check whether X days have passed from a certain date.

Answer 1

You can simply save the current date to a file when you close it. And when you open it, read it from the file and check it against the current date.

Answer 2

If the computer is disabled, then such a timer for writing to a file will not work. It's probably easier to rent the cheapest Wirth hosting and run a script that will notify you at the right time (send a message to the port, email or something like that).
Although we may not understand it that way, the question is very vague.
PS Previously (probably now - I was not interested) there were expansion boards for the motherboard in some servers with an Ethernet + RS232 on-board controller, which turned them on either by a timer or by a special UDP packet sent to the port.