C comment: mention 1500-02-29 as an invalid date

It is invalid because the Gregorian calendar is used for all years.

src/include/utils/datetime.h

@@ -274,7 +274,9 @@ extern const int day_tab[2][13];
 /*
  * These are the rules for the Gregorian calendar, which was adopted in 1582.
  * However, we use this calculation for all prior years as well because the
- * SQL standard specifies use of the Gregorian calendar.
+ * SQL standard specifies use of the Gregorian calendar.  This prevents the
+ * date 1500-02-29 from being stored, even though it is valid in the Julian
+ * calendar.
  */
 #define isleap(y) (((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0))