UTCDate calculations are not correct when dealing with DST - incorrect 1 hour offset

[cheryllaird]

I am trying to do some date manipulation on UTCDates, without having timezones get in the way. However when doing calculations that span over DST periods, I am finding the output has a 1 hour offset due to DST.

Starting UTC date:
2025-03-30T00:59:00.000Z
Action:
Adding 1 year (European timezone)
Expected result:
2026-03-30T00:59:00.000Z
Actual result:
2026-03-29T23:59:00.000Z

---

Starting UTC date
2025-03-30T00:59:00.000Z
Action:
Adding 3 months (Australian timezone)
Expected result:
2025-06-30T00:59:00.000Z
Actual result:
2025-06-30T01:59:00.000Z

Codepen reference:
https://codepen.io/cheryllaird/pen/azojbxX?editors=1010

[manfromanotherland]

Same problem here, did you find a solution?

[cheryllaird]

I have a workaround helper function for now, I run the result of every date calculation through this:

function correctDSTTimeOffset(newDate, oldDate, resolution) {
    const utcHour = oldDate.getUTCHours();
    const newUtcHour = newDate.getUTCHours();
    const hoursDiff = utcHour - newUtcHour;
    const validResolution = !["hour", "minute", "second", "millisecond"].includes(resolution);
    if (validResolution && Math.abs(hoursDiff) === 23) {
        newDate.setUTCHours(utcHour);
        newDate.setUTCDate(newDate.getUTCDate() + 1);
    } else if (validResolution && Math.abs(hoursDiff) === 1) {
        newDate.setUTCHours(utcHour);
    }
}

// Usage
const currentDate = new UTCDate(2025, 5, 3);
let newDate = add(currentDate , { month: 1 });

// We need to correct the new date if DST has been applied
correctDSTTimeOffset(newDate, currentDate, "month");