The Problem with Time & Timezones - Computerphile

That’s a hard question regardless of programming language. One commonly referenced YouTube video about time that explains why this is hard: [this video] A reasonable first approximation of a solution would be to just check every second (or minute, or hour, depending on requirements) whether the current system time is later than the scheduled time for any pending events. Then you probably want to make sure events are marked as completed so they don’t fire again if the clock moves backwards. Trying to predict how many seconds to sleep between now and 3pm on Saturday is a difficult task, but you can probably use a time library to do that if it’s important enough... but what happens when the government suddenly declares a sudden change to the time zone offset between now and then? The predictive solution would wake up at the wrong time.

Great question! Dates and times are one of the most loathed obstacles in writing software that needs to work a particular way across millions of devices situated all across the globe in different regions and time zones. In our case, we had a bit of code that aimed to measure the time until midnight/tomorrow. But when clocks change at midnight (jumping forward or backward), you can wind up with days that appear to be longer than 24 hours in length. And when a day is more than ~24hr in length, you wind up with unexpected answers to seemingly simple math equations. A bit of logic, responsible for setting up a timer, wound up scheduling numerous timers when our 'How long until tomorrow' code began returning negative numbers. That set of unintended timers locked-up the application itself. You can see our issue for the behavior here: [link] . For 10 minutes of anxiety, check out 'The Problem of Time & Timezones' from Tom Scott of Computerphile: [this video] . It's a fantastic summary of the issues programmers face when dealing with time :) Sampson (Brave Team)

It does not: timezone (TZ) definitions are politicial, inaccurate, and stupid and non versioned. The problem with TZ is TZ in its core definition. 1) they always change: if you have not updated your TZ since 3 months they are probably inaccurate 2) your TZ definition maybe accurate but they may not have been applied for real in the concerned zone 3) you can have different local time for the same longitude 4) you can have different days on the same longitude ... 5) TZ are not versioned, if the TZ changed between 2 records you may have made, you have inaccurate intervals stored. We do NOT have an API to take TZ change over time in consideration. 6) CEST/DST is breaking the axiom that time is a growing monotonic function References 8.5.3 TZ in postgresql man [link] Computerphile what's wrong with timezones [this video]

I'm not sure I follow (and this week, timezones and time handling are really important for my job, so I really want to understand)! If a user provides a time of 11/7/2021 1:50 AM America/New_York, how does the library determine if it's a -5 or -4 offset given that's when daylight savings time ends? I believe that's the ambiguity GP is referring to. I'm trying to align time across a few systems I work with. One provides a GMT time with an offset (clean and useful), another system reports timezones and local time, and yet another system has no idea when it moves between time zones, but reports time based on a single timezone consistently. Not dealing with multiple timezones in the past, it reminds me of Tom Scott's video on handling time and timezones, summarized at the end as roughly "Go find someone who built a library for handling times, thank them profusely, use their open source code, give them credit, and never worry about it again." [this video]