Add explanatory comment.

The 0-indexed month value may be confusing.

orbs.cpp

@@ -1607,6 +1607,9 @@ EX eItem targetRangedOrb(cell *c, orbAction a) {
 bool isValentines() {
   const time_t now = time(NULL);
   const struct tm *datetime = localtime(&now);
+
+  // 0-indexed tm_mon, 1-index tm_mday
+  // So this is February (2nd month), and the 14th day.
   return datetime->tm_mon == 1 && datetime->tm_mday == 14;
 }