...just an idea that I had for an alternative to time zones that doesn't have the issue of what day it is changing in the middle of the day. Not that I expect it to actually be used, since day and time keeping are fairly entrenched and reforms like this never catch on, and since probably not many people will see this anyways.
Times are always written in UTC±12:00, using a 24-hour clock. Dates are the same as they are currently, changing at midnight local time rather than at 0:00. After the time, the letter "E" or "L" is written. Between local midnight (when the day changes) and 0:00, the letter "E" (for "early") is used; between 0:00 and local midnight, the letter "L" (for "late") is used. This means that every date/time can be written in two different ways, depending on the time zone (e.g., 5:00 L Monday = 5:00 E Tuesday).
The time is always the same in all time zones.
The date can change at midnight like it does currently, rather than changing in the middle of the day for some time zones like it would if we used the simpler idea of just all using the same time zone and not changing anything else.
Time/date combinations written this way are unambiguous.
For time zones that are within a couple of hours of each other, the date and time will be the same for most of the day, and different only for a couple hours in the middle of the night.
Conversion between time zones doesn't require arithmetic beyond adding/subtracting one from the date or knowledge about the time zone you're converting from.
It allows more flexibility for when you switch to a different day; that is, people who are awake at midnight could choose a different time to switch dates without as many problems as the current system.
It's a new system, and those never catch on, plus it's fairly different from the current system
Some of the advantages are lessened in time zones near the International Date Line
It might be tricky remembering whether you need to add or subtract a day (but that's also a problem with the current system)
You can't get a good idea of what time of day it is (in terms of where the sun is and what people are doing) in an unfamiliar location just from the time
It doesn't quite work for time zones more than 12 hours away from UTC
Some of these problems can be solved by allowing EE, which is a day ahead of E, and LL, which is a day behind L.