Happily, you don’t need the linked code for your purpose.
in .items:
Date myDate ... {...}
String mySeason ... {...}
String myFullDate "Date: [%s]" //no Binding needed
in .rules:
rule "update myFullDate"
when
Item myDate updated //or
// Item mySeason updated // not needed, every time season changes, date changes too :)
then
myFullDate.postUpdate(myDate.state.toString + "(" + mySeason.state + ")") //mySeason should be string already
end
Now in sitemap you only need to use myFullDate.