[solardev]

To be honest, use a library where someone else figured out the ambiguities and accounted for the edge cases. Good starting point: https://moment.github.io/luxon/#/math

Date-fns is fine for simpler use cases but Luxon is a lot more complete, especially where it comes to time zones.

This is not the kind of thing you just want to blindly hack your way through without a really thorough understanding of how different cultures/locales/governments handle dates and times.

If you have to do math across daylight savings time boundaries that change over time (because governments and regulations change), converted to two or more time zones (which again have their own separate DST rules), and possibly span some 30 or 45 minute time zones (they exist!) this will quickly get out of hand and unreadable. Not just unreadable, but incredibly difficult to reason about.

It gets even worse when the backend stores only the offset (as in the ISO time, like T23:00:00+08:00) because then you lose the original time zone and can't be sure whether it came from a DST zone or another country in the same zone (but only during part of the year).