Most properly setup installation packages will set the installation date (IIRC you can't even disable it when using MSI).
You can detemine the date of installation for most software packages by looking at HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall\PACKAGE_GUID\InstallDate. It is a string value of YYYYMMDD. Microsoft Office GUIDs are formalized, and explained
here.
You can also use WMI and query for Win32_Product Try 'wmic product get installdate, name | grep --ignore-case office', or 'Get-WmiObject -query "SELECT Name, InstallDate FROM Win32_Product WHERE Name LIKE `"%office%`""' in PowerShell.
As far as getting it in LAN Sweeper, consider the following:
SELECT
tblComputers.Computer AS [Computer]
,tblSoftware.SoftwareName AS [Name]
,CASE ISDATE(tblSoftware.InstallDate) WHEN '1' THEN CAST(tblSoftware.InstallDate AS SMALLDATETIME) END AS [InstallDate]
FROM tblComputers
RIGHT JOIN tblSoftware
ON tblComputers.ComputerName = tblSoftware.ComputerName
WHERE
tblSoftware.SoftwareName LIKE '%microsoft%office%'
AND DATEDIFF(YEAR, CASE ISDATE(tblSoftware.InstallDate) WHEN '1' THEN CAST(tblSoftware.InstallDate AS SMALLDATETIME) END, GETDATE()) < 1
Here we use a CASE to check ISDATE on the date string and see if it is valid. If not, leave the field as NULL.