DATEDIF calculates the difference between two dates in terms of COMPLETED Days, Months and Years. Note the emphasis on COMPLETED.
It means that if elapsed years or months or days is 1.78, the answer would be 1 only. Hence, it gives only integer and doesn't round them.
A good documentation exists on following link
It is available in Excel by default. But it will not be available in Excel help. It will not be available in your list of functions in Excel. So, if you type =Date in Excel in any cell, below options get available -
But you can go ahead and use this as given on cpearson site.