In my 20 years of working with software and distributed architectures, no problems have been harder than working with time zones and calendars.
It is truly nightmarish stuff. I am convinced you could run a very profitable consultancy specializing in debugging date and time related problems in people's systems.
That would be profitable, but I'm not convinced anyone could stomach that job without going insane. Maybe a year if you were extremely hardy, but I'd be running for the padded cell after 3 months of dealing with nothing else.
It is truly nightmarish stuff. I am convinced you could run a very profitable consultancy specializing in debugging date and time related problems in people's systems.