Returns the number of days in the specified month in the specified year in the specified era.
year An integer that represents the year. month An integer from 1 to 12 that represents the month. era An integer that represents the era.
The number of days in the specified month in the specified year in the specified era.
For example, this method returns 28 or 29 for Nigatsu (February, month = 2), depending on whether year is a leap year.