The expanded syntax for the newly-implemented built-in function is as follows:
DATEDIFF( <timestamp_part> FROM <date_time> FOR <date_time> )
Use of the prepositions FROM and FOR as keywords conveys the wrong semantics. The meaning should be expressed by a semantically correct pair of prepositions.
DATEDIFF( <timestamp_part> FROM <date_time> TO <date_time> )
DATEDIFF( <timestamp_part> BETWEEN <date_time> AND <date_time> )
A contracted alternative syntax is available, sans prepositions:
DATEDIFF( <timestamp_part>, <date_time>, <date_time> )
As a temporary measure, the Beta 2 release notes will include a warning that the keywords will change in Beta 3 and recommending use of the contracted syntax for the time being.