It should be binary(8) according to the MSDN documentation
Values with the datetime data type are stored internally by the SQL Server 2005 Database Engine as two 4-byte integers. The first 4 bytes store the number of days before or after the base date: January 1, 1900. The base date is the system reference date. The other 4 bytes store the time of day represented as the number of 1/300-second units after midnight.
i.e. this round-trips:
select cast(convert(datetime, '2013-12-01 1:23 PM') as binary(8))
select cast(0x0000A14400DC8CF0 as datetime)