Modify DateTime for Application.Match - error 2042

  For w = 0 To FinalRow
    
    'Is open time less than close time?
    If (Cells(x, 4).Value + TimeSerial(0 + w, 0, 0)) < Cells(x, 6).Value Then    
           
    OpenTime_modified = Int(Cells(x, 4).Value + TimeSerial(0 + w, 0, 0))
           
        'RowNumber returns error 2402
        RowNumber = Application.Match(OpenTime_modified, Range("DateTime"))

    ...

If I remove where I highlighted in orange , "RowNumber" returns the correct position (which is "5").

My dates and time are mint. It's how I am trying to use Application.Match with the fact that I am trying to modify my date and time (e.g. "1/4/2016 3:00 AM") which is screwing it up?

Any ideas?