Axis.CategoryType (Word)

Returns or sets the category axis type. Possible return values are xlAutomaticScale - Word controls the axis type, xlCategoryScale - Axis groups data by an arbitrary set of categories, xlTimeScale - Axis groups data on a time scale.

You cannot set this property for a value axis.

With ActiveDocument.InlineShapes(1) 
 If .HasChart Then 
 With .Chart.Axes(xlCategory) 
 .CategoryType = xlTimeScale 
 .BaseUnit = xlMonths 
 End With 
 End If 
End With