By inserting cells with
Selection.Insert Shift:=xlToRight
you have moved your t1 range to the right. If those cells had values, they too would move to the right.
Maybe this will show you what goes on and what you can do. Step through the code with
F8 on the keyboard:
Code:
Sub monthprocess()
Dim t1 As Range, tNew As Range
Set t1 = Selection
Selection.Insert Shift:=xlToRight
Set tNew = Selection 'because the newly inserted range is selected
t1.Select
tNew.Select
End Sub