I wonder whether it would be better to use designated list-initialization (i.e. return {.tm_sec = _Seconds, /*...*/};) here.

Which standard mode is this part and which standard ode is this syntax?

Which standard mode is this part and which standard ode is this syntax?

Both are C++20.

Yes, _Fill_tm is defined in a #if _HAS_CXX20 scope (from line 2885); looks good.

I always forget that that's a feature, but I have no objections to its use (unlike abbreviated function templates). In a followup PR, if you want to do this.