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'
Since a datetime without a specified time segment will have a value of date 00:00:00.000, if you want to be sure you get all the dates in your range, you must either supply the time for your ending date or increase your ending date and use <.
select Date,TotalAllowance from Calculation where EmployeeId=1
and Date between '2011/02/25' and '2011/02/27 23:59:59.999'
OR
select Date,TotalAllowance from Calculation where EmployeeId=1
and Date >= ‘2011/02/25’ and Date < '2011/02/28'
OR
select Date,TotalAllowance from Calculation where EmployeeId=1
and Date >= ‘2011/02/25’ and Date <= '2011/02/27 23:59:59.999'
DO NOT use the following, as it could return some records from 2011/02/28 if their times are 00:00:00.000.
select Date,TotalAllowance from Calculation where EmployeeId=1
and Date between '2011/02/25' and '2011/02/28'