How to get the start date of an issue

Hi everyone,
I’m building a website that retrieve all the projects and the issues related to each one. My goal is to calculate if each project early, late or on time. For that I need to retrieve for each issue the start date and due date. I have no problem to get the due date with “{projectKey}” but I can’t find a way to get the start date of the issue. I already add start date in the task fields management.
If someone have an idea, I would be glad to hear it !