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

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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