Daylight savings time is hated by many, and twice per year a discussion reignites to get rid of it. Lot of folks feel this is a great idea. This year this decision seems especially close in the US. If this law passes, it will probably also change where I live 🇨🇦.

No doubt there’s lots of benefits and advantages to this, I don’t want to dispute that. This is also not an endorsement against that change, but I do however want to bring light to at least one disadvantage:

The timezone change was for a lot of developers a twice-per-year reminder that we need to use timezone databases and libraries.

This is useful, because every year many countries change their timezone rules. This means that if you schedule something in the future, say.. 2pm 6 months from, you don’t know yet with absolute certainty what UTC timestamp this will be. This is especially important when scheduling between people in multiple timezones.

The right way to handle this is to store the intended local time + a location such as America/Toronto. EST is not enough, because it’s EDT half the year, and obviously neither is -0500. I grew up in the netherlands, and it was only when I got into programming I realized that our timezone is not +0100 all year round, unlike what we learned in school.

Timezones are relatively stable in North America, but the US also made a change in 2007, and it sounds like we may have another one in the future!

So this bi-annual time change was a great reminder to many developers that timezones are a thing, and you can’t just naively assume a UTC time + an offset is enough. Even more so for teams that are spread cross-continent because the DST change doesn’t fall on the same day. Currently I’m in the 3 weeks per year the time difference between me and my parents is 5 instead of 6 hours.

A lot of programming is (seems?) anglo-centric. A similar situation is that before Emoji became wide-spread it was way more common to see a lot more issues around encoding non-ascii characters 🤷 (billpg). Especially in languages that don’t have good native unicode support (looking at you PHP).

So if DST goes away in North America, I predict we’ll see more people assuming using the offset is enough, resulting in bugs related to:

• Times in countries that have not yet abolished DST.
• Countries that ever change timezone rules. (This happens more often than you think!)
• Applications that deal with historical data.

It doesn’t help that one of the most common date formats (ISO 8601) uses an offset! (2022-03-18T17:05:30.996-0400). This is OKish for things that have already happened, but not good for anything in the future.

So when you hear developers excited about the US abolishing DST because it will make their (work) life simpler, remind them this is only true if you never intend your software to be used outside of North America, or when the entire rest of the world makes the same change and also freezes all timezone rules forever!