Every meeting in every outlook calendar in the country would have to change. Every exchange (equities, derivatives, commodities, etc) would all have to revise their trading schedules and applications, all the trading systems connecting to them would have to change and in some cases the actual laws governing those exchanges might need to be updated. The work contracts or at least the conditions of employment of millions of people would need to be changed. I've worked on applications that need to connect on a schedule to end points in other timezones, and plan and implement time and DST changes. It would be a nightmare.