Difference between two datetime entries

Difference between two datetime entries

SELECT ETA,STA FROM `Schedule` WHERE `Num`="5567";
2013-08-26 18:37:00 2013-08-26 18:30:00
SELECT DATEDIFF(ETA,STA) FROM `Schedule` WHERE `Num`="5567";
0
Why the result is 0 instead of 7 (i.e. minutes)?