Thank you for your reply.
When I display the gallery from the frontend, the categories are ordered alphabetically if this option is enabled in the administrator configuration. The 'Ordering' in the Category Manager does not change the way they are visible on the screen.
I use Joomgallery to do photo reports by a lot of users who can upload their photos.
We do a different activity every day and for each activity I created a unique category. We have about 350 a year and these reports remain visible for many years.
We do not have the possibility of making albums, and the only way to quickly find a category (report) is to order them by date of creation.
I don't know if this possibility is possible by modifying the php code a little.
Jean Pierre