> Maybe it makes sense to add that format along with dd/mm/yyyy, but that'll be in a future commit. I can do that
Actually, done - see [this commit](https://github.com/cousteaulecommandant/geany/commit/2dcf132466f194a29499a9c...) in my Geany fork (currently not included in this PR but it takes me zero effort to add it).
The menu with all six formats would look like this: 
As I said, I can include the commit with the extra formats in this PR (and change the title a bit), put it on a new one, or forget about it (so that we only have 4 formats).
> If people have asked for `dd/mm/yyyy`
Me, I have asked for it :D Well not really. To my knowledge, nobody has asked for dd/mm/yyyy nor mm/dd/yyyy, so if you prefer (and the sight of that screenshot terrified you) I can leave those out, and this PR is ready to merge.