Unfortunately, Excel's axis formatting dialog does not accept any kind of formula for the axis limits -- only number constants.

If you cannot trust your clients to be able to enter a suitable date/value into the axis minimum field, and you cannot trust Excel's "auto" algorithm to choose suitable axis minima/maxima, then you need to include some VBA code into the file that will determine the desired axis minimum value and enter that into the .minimumscale and/or .maximumscale properties of the desired axis. Do you have any VBA coding experience, or would this be your first attempt?