Sure, but back then there wasn't even a globally recognised calendar. Nor did they have a near-instantaneous global communications network, or a global governing body such as the UN. These days, if the UN decided to take such a step, they could give 10 years advance notice, and on the decided day everyone would just flick the switch. We've already shown that w are capable of managing this sort of change when we went through the y2k "bug" without any major dramas, despite this requiring a large percentage of the world's information systems needing to be updated / tested. So I personally wouldn't anticipate any significant friction after the UN signed-off.
The Y2K bug is a very different case. That's a case in which the calendar system was perfectly able to deal with reality, but some people had buggy code implementing it. All it took was those people fixing their bugs, mostly completely independently, not some big coordinated effort.
As for y2k, you know what, I could have stuck a load of different examples in there. Changes to http for example. Or how about the switch from national currencies to the Euro in Europe. Each one is different, but they all demonstrate that important changes can be made to large interconnected systems, and that the change can be activated at the flick of a switch if the need for synchronisation exists. Also, I assume you're too young to know much about y2k if you can talk so blithely about people working independently in an uncoordinated manner. Y2k was huge - if you change one system to requiring 4 digit years, but just one of the systems it communicates with requires two digit years, your system stops working. Both need to switch to 4 year dates on that interface simultaneously. When the systems you're talking about are command and control circuits, financial transfer systems etc, this was a major issue.
You might be able to get Europeans to transition, you might even get the whole rest of the world to transition, but without the United States, it's not truly a universal system.
The UN doesn't have that authority, either politically or morally.