How to run a function at specific time & date?

Question

How can I run a function at a given time and date?

Example: I have a function that needs to run on the 12th of each month at 10AM.

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Answer 1

It's not advised to use setInterval because it has non-deterministic behaviour - events can be missed, or fire all at once. Time will fall out of sync, too.

The code below instead uses setTimeout with a one minute period, where each minute the timer is resynchronised so as to fall as closely to the hh:mm:00.000s point as possible.

function surprise(cb) {
    (function loop() {
        var now = new Date();
        if (now.getDate() === 12 && now.getHours() === 12 && now.getMinutes() === 0) {
            cb();
        }
        now = new Date();                  // allow for time passing
        var delay = 60000 - (now % 60000); // exact ms to next minute interval
        setTimeout(loop, delay);
    })();
}