SQL date error

i have an SQL query:
SELECT SUM(TotalHours)
FROM Activity
WHERE Date
BETWEEN #11/10/00# AND #13/10/00#;
the query gives me a result of 256 when upon review of the data the result should be 50 (10 entries of 5 hours each).
Can anyone suggest a theory as to why this error in calculation is happening.
Cheers,
ghost.