:I take it back. It turns out the Tenpō_calendar used from 1844 to 1872 had months with either 29 or 30 days. This is another very likely reason. So in 1871 (or Meiji 4 at that time), there indeed was a date of 2/29. It is still a question of whether this date (and many others taken from Japanese sources) have been taken after they were adjusted to the Gregorian calendar (and every date was pushed ahead about 50 days to match with the Gregorian calendar when they changed it over).