The edit dialog time picker didnt take into account GNOME's hour format.
The edit dialog time picker didnt take into account GNOME's hour format because the desktop settings clock-format from gcal-window was not passed to gcal-edit-dialog.
Now the data is passed through the function gcal_edit_dialog_time_format and everything works as required.
https://bugzilla.gnome.org/show_bug.cgi?id=703321