Call subroutine from another workbook

Is it possible to call a subroutine from another workbook?

I have a subroutine called TestSub in my "Test Book.xls" workbook. I open this book, then try to call subroutine (TestSub) in that book.

    If frmDate.chkATime Then
        Set wb = Workbooks("Test Book.xls")
        If wb = Nothing Then
             Workbooks.Open "C:\Test Folder\Test Book.xls", False, True
        End If
        Call TestSub
        Workbooks("Test Book.xls").Close False
    End If

Is this even possible?

TIA,
DejaVu