Setting the day property with a value lower or higher will throw an error.
Datetime.day read/write property
The Datetime.day property returns a number representing the day of the month. Valid values are from 1 through 31.
Setting the day property with a value lower or higher will throw an error.
Example
-- prints the current day, for example "1" for the first day of the month
print(sys.Datetime().day)