Marmotte73,
I’d be happy to help you out with this. The Community Submission form uses your WordPress date/time settings. So, just go to your main WordPress settings and change your time format to 24h, like this: http://d.pr/i/t0Nt
Just let me know if that does the trick for you. Thanks!
Great! Since it’s looks like this got you sorted, I’m going to mark this thread “Answered” and close it out.
We really appreciate your support and please let us know if you need anything else in the future. If you find yourself with a few minutes and would be so kind as to give us a “Works” rating or a positive star voting on the WordPress.org repo (http://wordpress.org/extend/plugins/the-events-calendar/), or even a shoutout to your followers on Facebook or Twitter, it would mean a lot to us. Thanks again for using the plugin.