Tracking date when a drop down value is selected

I'm creating a project tracker where I can assign what phase the project is at (design, prototype and complete). What I want to do is track how long the project is in each phase, so what I plan on doing is tracking when the drop down items are changed. I thought of using an IF statement where if "design" is selected, then I mark today's date in the "design" column. As you can see from my pictures, once I select the new phase, it erases the date I had in the "design" column. Is there a 'do nothing' function for an IF statement, or a work-around for this without buying an add-on?