If a "Date Time" literal in the SDL file has a time zone that's not found in
your system, you get one of these instead of a SysTime. (Because it's
impossible to indicate "unknown time zone" with std.datetime.TimeZone.)
The difference between this and DateTimeFrac is that DateTimeFrac
indicates that no time zone was specified in the SDL at all, whereas
DateTimeFracUnknownZone indicates that a time zone was specified but
data for it could not be found on your system.