Post-dating records isn't easy and given the current move to locking records it isnt likely to get any easier either...That being said ...if you are using a normal workflow for setting visits it automatically creates a visit entry.  If you later need to go back and and a new note ...when click the visit and the add the note and it will automatically appear under that visit...you will need to adjust the date of the note.  Although argueably the date of the note is accurate as it is the date you entered the information ....where as the visit represents the day the animal visited.