goog.date.UtcDateTime.fromIsoString does not support extended year

[p-himik]

According to a reference of Date.toISOString, the format is either YYYY-MM-DDTHH:mm:ss.sssZ or ±YYYYYY-MM-DDTHH:mm:ss.sssZ.
However, only the former is supported:

(new Date(-1, 1)).toISOString()
=> '-000001-01-31T21:46:32.000Z'
goog.date.UtcDateTime.fromIsoString((new Date(-1, 1)).toISOString())
=> null

[12wrigja]

For what it's worth, you may find your date/time manipulation needs better solved by Temporal, for which there are several polyfills available.

[shicks]

We also have an issue with the formatting - it should always pad the year to 4 unsigned or 6 signed digits, and currently it does not.