At some point you will need to calculate how many leap years to account for and whether or not a leap second was introduced in between two arbitrary times.

Time libraries can help you with that.

Exactly there is the same problem, to a lesser extend, when adding/subtracting years which is not trivial to do properly.

Let alone determining requirements for thresholds.

This discussion reminds me of this bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1715455

Why can’t you use Date.getFullYear with setFullYear?

Absolutely. moment has a definition of that and compute it in a consistent way, which is useful. But if you want to come up with a different definition then it cannot help you indeed.