system
6
Well, the daylight saving time switching could be a real pain in the global world 
While most Northern hemisphere countries switch to (and from) it - and even in the same time (it was not true several years ago), there are many countries (especially closer to the equator) which do not apply this. And some countries in the Southern Hemisphere switch the time in exactly opposite directions! So there is no simple way how to do this globally.
Of course - if your computer serves just one country or region, it is much simpler, but even there you can find some problems: take Arizona as an example - it is the only US state which does not use the daylight time so during part of the year it is 1hr shifted comparing to the other stats around.
Hope this helps
Pavel