Columns("D:D").Select
Selection.TextToColumns Destination:=Range("D1"), DataType:=xlDelimited, _
TextQualifier:=xlDoubleQuote, ConsecutiveDelimiter:=True, Tab:=False, _
Semicolon:=False, Comma:=False, Space:=True, Other:=False, FieldInfo _
:=Array(Array(1, 4), Array(2, 9)), TrailingMinusNumbers:=True
It loses the time element.
To include time:
With Range(Range("D2"), Cells(Rows.Count, "D").End(xlUp))
.TextToColumns Destination:=Range("D2"), DataType:=xlDelimited, _
TextQualifier:=xlDoubleQuote, ConsecutiveDelimiter:=True, Tab:=False, _
Semicolon:=False, Comma:=False, Space:=True, Other:=False, FieldInfo _
:=Array(Array(1, 4), Array(2, 1)), TrailingMinusNumbers:=True
.NumberFormat = "mm/dd/yyyy hh:mm"
For Each cll In .Cells
cll.Value = cll.Value + cll.Offset(, 1).Value
Next cll
.Offset(, 1).Clear
End With