Sub Application_Quit() clears the value of defined variable

I'm trying to pop up a message box when Outlook is closed that tells what time Outlook was opened and what time it was closed.

Private startTime As String
Private endTime As String

Private Sub Application_Startup()
    startTime = CStr(TimeValue(Now))
End Sub

Private Sub Application_Quit()
    endTime = CStr(TimeValue(Now))

    MsgBox _
        "Session started at " + startTime + vbNewLine + _
        "Session ended at " + endTime, _
        vbOkOnly + vbInformation, _
        "Session Information"
End Sub

This is what I expect:

My variables are cleared when Application_Quit() is triggered.
I receive this message box instead:

I know that startTime is being given a value on Application_Startup(). When I add the following line of code to the end of Application_Startup() I receive the below message box.

    MsgBox _
        "Session started at " + startTime, _
        vbOkOnly + vbInformation, _
        "Session Information"

Why does Application_Quit() force startTime = "", and is there any way around it?

Using FSO to create a temporary text file removes the need for Global variables startTime and endTime as their values are stored in the temporary file and recalled during Application_Quit().

Private FSO As Object
Private oFile As Object

Private Sub Application_Startup()
    Set FSO = CreateObject("Scripting.FileSystemObject")
    Set oFile = FSO.CreateTextFile("C:\Users\username\Documents\Outlook Files\temp.txt")

    oFile.WriteLine "Session started at " + CStr(TimeValue(Now))
    oFile.Close

    Set oFile = Nothing
    Set FSO = Nothing
End Sub

Private Sub Application_Quit()
    Set FSO = CreateObject("Scripting.FileSystemObject")
    Set oFile = FSO.OpenTextFile("C:\Users\username\Documents\Outlook Files\temp.txt")

    MsgBox _
        oFile.readline + vbNewLine + _
        "Session ended at " + CStr(TimeValue(Now)), _
        vbOKOnly + vbInformation, _
        "Session Information"

    oFile.Close
    Set oFile = Nothing
    FSO.DeleteFile ("C:\Users\username\Documents\Outlook Files\temp.txt")
    Set FSO = Nothing
End Sub