Using a formula column that results in a date, that can be used in a gantt chart

I created a Formula column. The formula I used is an IF statement that returns a date if true. The issue I’m having is that, when I go to the Gantt chart view, my Formula column does not appear in the choices of “Choose timeline columns”. Is there something I’m missing? Thanks!

I posted the original question a couple days ago. Is there a moderator that can answer?