You don't. Instead, you can directly access the underlying properties in the property tree.
Really, this isn't complicated at all - all variables in FlightGear will at some point end up in the property tree, that is where things are stored and can be accessed/read and modified later on.
So, if you don't have any background knowledge at all, the easiest thing to get started would be looking at similar (or possibly even identical features) in FlightGear and simply check what properties are being used. This could be done by inspecting the source code, or -in your case- by simply looking at the "time of day" dialog in $FG_ROOT/gui/dialogs: http://gitorious.org/fg/fgdata/trees/master/gui/dialogs
Specifically, you need to have a look at timeofday.xml: http://gitorious.org/fg/fgdata/blobs/ma ... eofday.xml
There, you can see all properties being used.