From MS SQL documentation:

datetime: January 1, 1753, through December 31, 9999

smalldatetime: January 1, 1900, through June 6, 2079

Note:

datetime is stored as two 4-byte integers for days from 1/1/1900 and milliseconds since midnight.

smalldatetime is stored as two 2-byte integers for days from 1/1/1900 and minutes since midnight.