For me I wanted a value between 2008 and 2009 (one year: 60 seconds * 60 minutes * 24 hours * 365 days = 31536000). Because the Unix timestamp doesn’t support fractions your need to round the value to an int. (floor or round the value).
Code:
SELECT FROM_UNIXTIME( UNIX_TIMESTAMP('2008-01-01 01:00:00')+FLOOR(RAND()*31536000)