VBA has native Date functions as well as a Date data type that allow for comparing and storing date information. The DateAdd function returns a future date (or previous date using a negative interval) using the specified interval. The interval can be in seconds, minutes, hours, days, months, weeks, or years. The DateDiff function will return the difference between two days in the specified interval.

The example below calculates a target date 180 days prior to the current date then finds all files in the specified folder that were last modified prior to the target date.

Sub Test()
    ArchiveFiles ActiveDesignFile.Path, 180
End Sub

Sub ArchiveFiles(strPath As String, numDays As Long)
    Dim dateTarget As Date
    Dim iDays As Long
    Dim strFile As String

    ' Target date is numDays before today
    dateTarget = DateAdd("d", numDays * -1, Date)

    ' find all files in current folder
    strFile = Dir(strPath & "\*")

    Do While Len(strFile) > 0

        ' get the number of days since the file was created or last modified
        iDays = DateDiff("d", FileDateTime(strFile), dateTarget)

        ' If older than numDays send to archive

        If iDays > numDays Then

            ' Archive file

        End If

        strFile = Dir

    Loop

End Sub