With 3.15 SP6 the ruler window is a real panel, so you can adapt your case (1) as you like. Check trendRulerPanel.pnl
The format used in your case (2, dat/time displayed at the ruler inside the plot area) is hardcoded to use the locale information according to the projects current language
(so it uses %x and %X).
In WinCC OA 3.14 you cannot change the time format for the trend ruler popup.
As Martin Koller has written the trend is using the hardcoded formats "%x" for the date and "%X" for the time.
It probably will work if you change the settings in the operating system how the date/time shall be displayed for a specific locale.
Note: All applications running on this computer will probably be affected.
Best Regards
Leopold Knipp
Senior Support Specialist