[coder543]

That’s a hard question regardless of programming language. One commonly referenced YouTube video about time that explains why this is hard: https://youtu.be/-5wpm-gesOY

A reasonable first approximation of a solution would be to just check every second (or minute, or hour, depending on requirements) whether the current system time is later than the scheduled time for any pending events. Then you probably want to make sure events are marked as completed so they don’t fire again if the clock moves backwards.

Trying to predict how many seconds to sleep between now and 3pm on Saturday is a difficult task, but you can probably use a time library to do that if it’s important enough... but what happens when the government suddenly declares a sudden change to the time zone offset between now and then? The predictive solution would wake up at the wrong time.

[vlovich123]

No, you say "sleep until 3pm on saturday" you don't predict anything. The OS computes an exact value to wake up in when you arm. If that clock then jumps forwards or backwards or does anything weird you then recompute the expiry timeout for that timer. You can't do this in app-space but AFAIK all OSes provide a facility for you to do so.

https://developer.apple.com/documentation/dispatch/1420517-d... http://man7.org/linux/man-pages/man2/clock_nanosleep.2.html