SQL Query date / datediff

you may want to read up this article about date/time stuff:
https://www.experts-exchange.com/Database/Miscellaneous/A_1499-DATE-and-TIME-don%27t-be-scared-and-do-it-right-the-first-time.html

hence, syntax like this:
WHERE    dateDiff(dd, SMS_G_System_OPERATING_SYSTEM.InstallDate, GetDate()) < 30
should be banned ...
the correct way would be for example:
WHERE    SMS_G_System_OPERATING_SYSTEM.InstallDate > dateadd(day, 30, GetDate())

now, getdate() contains also a time portion (data type is datetime), so you may want to use the date data type instead: CONVERT(date, GETDATE())
http://technet.microsoft.com/en-us/library/ms188383.aspx

for your query, it would look like this:
WHERE    SMS_G_System_OPERATING_SYSTEM.InstallDate > convert(date, getdate())

You should use table alias names to improve readability. E.g.

SELECT  S.ResourceID ,
        S.ResourceType ,
        S.Name ,
        S.SMSUniqueIdentifier ,
        S.ResourceDomainORWorkgroup ,
        S.Client
FROM    SMS_R_System S
        INNER JOIN SMS_G_System_OPERATING_SYSTEM OS ON OS.ResourceId = S.ResourceId
WHERE   DATEDIFF(DAY,OS.InstallDate, GETDATE()) = 1;

Select allOpen in new window

btw, there is no "forward"..

WHERE
    SMS_G_System_OPERATING_SYSTEM.InstallDate >= CONVERT(char(8), GETDATE(), 112)