Start dates, end dates and # of Days

Preface: I am working on a template scheduling file for construction management. I have Suretrack and know about Microsoft Project but I want a something specific (functionally and aesthetically) and simple (once it is locked).

Problem: I want to make it so that there are three columns where a Start Date, End Date, and number of days can be entered.

Depending on which column the user chooses to place input (minimum 2 input values), I want the other (3rd) value to be calculated. I know how to do each individually: If the number of work days are provided, I simply add that value to the start date. If the end date is provided, I use the networkdays function to determine the number of workdays. My problem is that I need to make it user friendly for someone using the template.

I want the user to choose the input (maybe with a drop-down menu?) and have the other value calculated.

Thanks for your help!