SQL是否自动计算每月开始和结束日期？

我试图获得1个月内售出的总销售额。例如，从15年4月1日至15年4月30日。

我的跨度为3/30/15-4/30/15。

这是我的SQL：

    SELECT  Customer.custno
      , Customer.enteredDate AS 'Date Entered'
      , COUNT(BasicPolInfo.polid) AS 'Number of Policies'
      , SUM(COUNT(BasicPolInfo.polid)) OVER (ORDER BY Customer.custno ROWS 
    BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS TotalAmount
    FROM    Customer
        INNER JOIN BasicPolInfo ON Customer.custid = BasicPolInfo.custid
    WHERE   BasicPolInfo.polid IS NOT NULL
        AND Customer.firstname IS NOT NULL
        AND Customer.enteredDate BETWEEN DATEADD(MONTH, -1, GETDATE()) AND
        DATEADD(MONTH, 0, GETDATE())
    GROUP BY Customer.custno
      , Customer.firstname
      , Customer.lastname
      , Customer.entereddate
    ORDER BY Customer.enteredDate ASC

我得到的结果是从2015-04-30到2015-03-30的结果。我正在尝试将2015-04-30升级为2015-04-01。

下个月也是2015-05-31至2015-05-01

任何帮助，将不胜感激！谢谢！