Find next occurrence of a time in a non-local timezone (in this case, PST)

Question

I want to get how far away is the next occurence of a particular PST time regardless of the client's timezone.

This would be trivial if the time were in UTC but I don't know how to do it in PST keeping in mind the observance of daylight savings time.

Eg. 4 PM PST would be 11 PM UTC since it is right now summer.

I would prefer not to have to manually input the dates of daylight saving time.

I am happy to use a library if this is not possible without one.

// returns the number of milliseconds from the current time until the specified time in PST.
function getTimeUntil (hour, minutes = 0, seconds = 0)
{
    // implementation needed
}

Assuming PST is US Pacific Standard Time, then 4 pm (16:00) PST is 00:00 UTC always, since PST has a fixed offset: UTC -8. Most (though not all) places that observe PST in winter observe PDT (UTC -7) in summer. In those places 16:00 PDT is 23:00 UTC. — RobG, Jul 17 '21 at 10:49

score 1 · Answer 1 · answered Jul 17 '21 at 10:49

The following is an explanation of why this is likely a duplicate of How to initialize a JavaScript Date to a particular time zone.

PST (presumably US Pacific Standard Time) is a timezone with a fixed offset, UTC -8. Places that observe PST and have daylight saving typically call that offset Pacific Daylight Time (PDT), which is UTC -7.

PST might also be Pitcairn Standard Time, which is also UTC -8 and observed all year round on Pitcairn Island. Converting PST to UTC is achieved by adding 8 hours.

However, likely you want to work with times and dates for a place that observes US PST in winter and US PDT in summer, e.g. Los Angeles. In that case you can use a library like Luxon or date.js that allows creating dates based on a timestamp and specified IANA representative location such as "America/Los_Angeles". If that is the case, then see the link above.

I'm not sure where on pacific standard time daylight savings time is not considered but I'm pretty sure that in Canada and the US it is. — trinalbadger587, Jul 17 '21 at 16:24

score 0 · Accepted Answer · answered Jul 17 '21 at 17:04

My implementation:

// returns the formatted time from the current time until the specified time in PST.
function getTimeUntil (hour, minutes = 0, seconds = 0)
{
    let future = luxon.DateTime.now().setZone('America/Vancouver').set({
        hours: hour,
        minutes: minutes,
        seconds: seconds
    });
    let now = luxon.DateTime.now().setZone('America/Vancouver');
    if (future < now)
    {
        future = future.plus({ days:1 });
    }
    return future.diff(now, ["hours", "minutes", "seconds"]);
    // implementation needed
}

console.log(getTimeUntil(13, 0, 0).toObject());

<script src="https://cdn.jsdelivr.net/npm/luxon@2.0.1/build/global/luxon.min.js"></script>

Find next occurrence of a time in a non-local timezone (in this case, PST)

2 Answers2