AFAICS from 9899 1998 Draft to 2023, the *mktime* wording has been essentially:

"The original values of the tm_wday and tm_yday components of the structure are 
ignored, and the original values of the other components are not restricted to 
the ranges indicated above. On successful completion, the values of the tm_wday 
and tm_yday components of the structure are set appropriately,"

so the recommendation has been to do something like (pointless example):

	time_t tt = time(&tt);
	struct tm *tm = localtime(&tt);
	tm->tm_wday = tm->tm_yday = -1;
	if ((tt = mktime(tm)) == -1 && tm->tm_wday == -1 && tm->tm_yday == -1)
		error(...);

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry