libc: ignore '?' in date format strings

Ignoring '?' when formatting date strings allows
the format strings to be reused for parsing. This
is convenient, since we don't need to duplicate
the format strings.

sys/src/libc/port/date.c

@@ -428,6 +428,9 @@ static int
 		switch(c0){
 		case 0:
 			break;
+		/* Ignore '?' so we can share parse and format strings */
+		case '?':
+			continue;
 		case 'Y':
 			switch(w){
 			case 1:	n += fmtprint(f, "%*d", pad, tm->year + 1900);		break;