Hi, Iorna. Sorry you’re experiencing this.
I visited http://lovewimbledon.org/whatson/, which defaults to Photo View. I saw it did have the events out of order. I visited your List View and Month View as well, and they seemed to be in the correct order.
You can see our demo site’s Photo View is not displaying the events out of order: http://wpshindig.com/events/photo/
I suspect your Photo View issue has to do with your theme or a code customization, if you’ve made any.
Since this isn’t currently a known issue, I’d suggest following these recommended troubleshooting steps for your own site:
Please follow our Testing for Conflicts Guide (basically switch to Twenty Seventeen theme and deactivate all plugins and custom code other than Modern Tribe plugins) and see if that helps narrow down the cause of this.
If it doesn’t, please enable WP_DEBUG and WP_DEBUG_LOG (which will create a file on your server at /wp-content/debug.log if there are any WP_DEBUG messages) and share any debug messages you see while trying to replicate this issue and doing other things on your site relevant to this ticket (such as visiting your site’s home page, events page, single-event pages, and anything else you can think to do).
Then, please re-share your System Information (while in Testing for Conflicts Mode) in a Private Reply. That will give me a lot of extra information to help troubleshoot this.
Let us know what you find out.
Thanks.