You need to convert the date field to varchar to strip out the time, then convert it back to datetime, this will reset the time to '00:00:00.000'.
SELECT *
FROM [TableName]
WHERE
(
convert(datetime,convert(varchar,GETDATE(),1))
between
convert(datetime,convert(varchar,[StartDate],1))
and
convert(datetime,convert(varchar,[EndDate],1))
)