The most recent one, the transition from the Julian calendar to the Gregorian calendar, lasted for about 400 years, as different countries transitioned at different times, with, I'm sure, plenty of friction caused by the lack of synchronization.
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 UN is not a "global governing body." It has no real power; and it doesn't deal with things like calendar standards. And it would take a lot more than "sign off" from some global standards body to get people to actually switch. It would require completely re-writing every piece of date handling software ever.
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.
Sigh. You know what, I actually tried to find a better term than "governing body" but at the end of the day, I didn't really find an alternative that wouldn't immediately get some peanut objecting to its exactness, so I went with governing body. Thankyou for being the peanut. So, international politics 101. The UN is a body whereby the nations of the planet can get together and discuss global problems and their solutions. This type of body did not exist 1500 years ago. Yes, like any reasonably well-educated adult on the planet I'm aware that the UN can't make laws, and couldn't enforce them if it did. But in the real world there are other means available to the UN to pressure member states to do the right thing. If 99% of nations voted for a new calendar, the remaining 1% are going to look pretty silly if they stay on the old calendar.
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.
None of which ultimately matters. The United States, which for better or worse has significant influence on the world (UN included) could not even switch to the metric system on a time frame and manner of its own choosing. How well do you think we could transition from one system of time- and date-keeping to another, which is arguably more integral to people's lives than other types of measurement, especially when done by fiat through an external body?
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.