Although the source for the fourth edition is not available, the man page for the ctime(3) library function is:
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V4/man/man3/ct...
It confirms that the day of the month element was one based. This function is different in earlier editions:
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V3/man/man3/ct...
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/man/man3/ct...