Good afternoon Hélène and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
With that in mind, I would recommend going to your WordPress backend Settings > General and perhaps taking a look at the hour format available on your installation.
If that does not work, there is a good chance that the time format is embedded in your WordPress theme.
Let me know how that goes.
Best regards,
Geoff B.