This command changes the day of the month in-game (1-31).
set gameday to [day]
|Day||The day of the month you wish to set the day to - 11 would set the date to the 11th of the current month. Note: this will break your save if you set the day to a non-existent number (e.g. 31 in February).|
set gameday to 15
This command would change the date to the 15th of the current month.