I don't think the tooltip can be formatted, unfortunately. The clocks in Fatdog64 812 and 901 are two different plugins, with different capabilities. The LxQt team discontinued the old clock plugin in favor of the worldclock plugin. One can see in source code commit [1], which added a tooltip to worldclock, that the format is fixed. In commit comments they added "Work in progress, add customizable tooltip format" but their intention remains unfulfilled to date.
Not what you asked, but to change the visible time/date in the panel, right click the clock icon and select configure "World Clock" to access the worldclock plugin configuration dialog. If you need advanced control of the time format tick "Advanced manual format" in the "Display format" tab then click button "Customise...". Follow instructions in the new dialog.
See also
[1] Commit introducing tooltip: <https://github.com/lxqt/lxqt-panel/comm ... ee732226d8>
[2] Fatdog64 901 worldclock plugin settings source code <https://github.com/lxqt/lxqt-panel/blob ... ration.cpp>.