C言語での月が「0月から11月」ってアホな仕様になった最初と思われるソースコード、現存のコードの範囲で調べる限りUNIX V5のこれっぽい?><(V2ではアセンブラで書かれててV3とV4とはソースコード失われてる><(V3は当該部分はアセンブラのままっぽさ?><))
https://mstdn.nere9.help/@orange_in_space/107182710429039984
V5/usr/source/s3/ctime.c
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s3/ctime.c
* day of month (1-31)
* month (0-11)
・・・・・・・・・・><
V9/libc/gen/timec.c
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V9/libc/gen/timec.c
static char Month[][3] = {"jan","feb","mar","apr","may","jun",
"jul","aug","sep","oct","nov","dec","\0"};
V9/include/time.h
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V9/include/time.h