Invalid dates in Japanese eras?

[Edward Welbourne]

Wáng Yifán (09 September 2021 05:07) wrote:
> [...] I couldn't find the exact definition of "extended Gregorian year" elsewhere.

My semi-educated guess is that "extended Gregorian year" means the year
that the relevant date would fall in, in the "extended Gregorian
calendar" - i.e. the Gregorian calendar extrapolated backwards, if
necessary, to before Gregory introduced it.

	Eddy.