Most do, but the problem is less about the library and more about understanding how to use it. For example, if you want an event to happen in 12 hours, and it

Most do, but the problem is less about the library and more about understanding how to use it. For example, if you want an event to happen in 12 hours, and it happens to be 9pm the day before daylight savings time, do you schedule it using the timezone-aware API (so it happens at 9am, which is actually 13 hours away) or 8am (which is 12 hours, but non intuitive)? What about the event that's supposed to happen every 12 hours in perpetuity? What happens when the user/device is mobile, and crossing timezones? Which times do you use? What happens when you're scheduling something far in advance, and then the timezone definition itself changes (as happens a few times a year) between the time you scheduled the event, and the time something actually is supposed to happen? Does the event adjust for the new definition or follow original time? Luckily for many problem domains, the details around this don't matter too much, but this is just the tip of the iceberg with timezone challenges.