DATEDIFF is a SQL Server function that calculates the difference between two dates or datetimes, returning an integer value. The syntax involves specifying the unit of time (year, quarter, month, day, hour, minute, second) and the start and end date values. DATEDIFF returns only positive integers, so if the end date is earlier than the start date, a negative result is obtained. It calculates the number of specified datepart boundaries crossed between the two dates but does not count the units between them. The function's accuracy can be limited due to truncation or rounding issues, and it requires consideration of boundary values when calculating differences in years or months. DATEDIFF is useful for finding the age of objects, calculating time intervals, and analyzing data. It has equivalents in other database systems like Snowflake, MySQL, and PostgreSQL, although with some variations in supported datepart values.