you should put those two dates between single quotes like..
select Date, TotalAllowance from Calculation where EmployeeId = 1
and Date between '2011/02/25' and '2011/02/27'
or can use
select Date, TotalAllowance from Calculation where EmployeeId = 1
and Date >= '2011/02/25' and Date <= '2011/02/27'
keep in mind that the first date is inclusive, but the second is exclusive, as it effectively is '2011/02/27 00:00:00'
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