X-axis trouble

I'm using the below code to try to define the min and max of the x range on a chart with two numbers that I have dynamically named in a cell (Chart_Date_Pmonth and Chart_Date_Tmonth). These numbers change over time. However, I cannot get the code to accept these named variables.

    ActiveChart.Axes(xlValue).Select 
    Set ActiveChart.Axes(xlValue).MinimumScale = Chart_Date_Pmonth 
    Set ActiveChart.Axes(xlValue).MaximumScale = Chart_Date_Tmonth