The calendar model that ships with FlexGantt allows the application to define
which days, hours, minutes have some kind of semantic to them. The default 
calendar for instance defines that Saturday and Sunday are to be considered
weekend days. This causes the timeline to use a different text color when 
drawing the text of a weekend day. The layer container below the timeline uses
the same model to gray out the background of weekend days. The layer container
doesn't even know that Saturday and Sunday are weekend days, actually it doesn't
even know what a weekend is. All it knows is that the calendar model returned
a list of calendar entries for a given time span and that it can use a color map
to determine which color to use for which type of entry.
