We don't - and in fact can't use the same dates, though. Right now, where I am, it's still Tuesday, May 29 - but in Europe, at this very same moment, it's already May 30 - a Wednesday. Maybe you would argue that we force everyone to run their days midnight to midnight on the same calendar - so instead, I would run on the European calendar, and my workday would have begun on May 29, but ended May 30. I guess today was a Tue-wednes-day? That's all very well, except now we have the weird situation of the US workweek running from Montueday through to Frisatday - while the Australian workweek, if they followed the same approach, would go from Sunmonday through to Thufriday.