How to get Dates to work in base parameter?

Hi everyone, i’m trying to make a gantt like chart but i can’t make dates work in base parameter, any ideas on the matter?

Here is a pen to demonstrate : https://codepen.io/Patoulache/pen/jOENOVM

I found a solution by changing the date in int with the timeStamp to make it work.
Guess i got a lot more things to learn.