Display days passed in decimal or integer in Ms SQL 2005 script

The script like a Accounts Receivables script:

DECLARE @ReportDate AS DATETIME
DECLARE @TimePass AS DECIMAL
SET @ReportDate = '20111231'
SET @TimePass = 30

SELECT FirstDate, LastDate, (LastDate-FirstDate)   FROM RM20101
WHERE LastDate <=@ReportDate
AND   (LastDate-FirstDate) > @TimePass

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The problem is that the value displayed by "(LastDate-FirstDate)" is in Date Format.  Example (noticed the difference column):

FirstDate               LastDate                Differences
----------------------- ----------------------- -----------------------
2016-02-20 00:00:00.000 2016-03-22 00:00:00.000 1900-02-01 00:00:00.000
2016-03-27 00:00:00.000 1900-01-01 00:00:00.000 1783-10-07 00:00:00.000
2014-01-21 00:00:00.000 2014-02-20 00:00:00.000 1900-01-31 00:00:00.000

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Where is the script going wrong?

Thank you very Much!!!!!