What the title says. I want to join my first jam and am just curious how they do this. Thanks! 

When you join a jam, the date your files are uploaded is displayed below their filename. Additionally, your game’s More information section shows when you page is originally published. Although we don’t have an automated check right now (we will be adding it), a jam host can go to your page and see if you’ve created the page or uploaded files before the jam has started.