How to Think About Time in Programming

I never really took time seriously until one of my cron jobs skipped execution because of daylight saving. That was the moment I realized how tricky time actually is.

This article explains it really well. The part about leap seconds especially got me. We literally have to smear time to keep servers from crashing. That’s kind of insane.

I avoid running "daily" cron jobs or other scheduled tasks around 2am for that reason, they might not get run or them might get run twice.

Where practical I schedule them around 12:00 (but I'm sure one day I'll get stung but some odd country who chooses to implement their daylight savings changeover in the middle of the day).

The issue with leap seconds is that the BIPM recommends that everyone should use UTC for their "source of truth" fundamental representation for time stamps, so pretty much every software system does that. That's the core mistake.

Everyone should use TAI as their fundamental representation. TAI has no leap seconds. It's way easier to convert from TAI to UTC than vice versa. You can still easily present all your timestamps in UTC when printed as a string.

NTP servers are generally synced up to GPS signals, which already use a version of TAI for their time signals. So an NTP server will take a perfectly good TAI time signal and do a smearing conversion to something that looks more like UTC (but isn't, because a true UTC clock would occasionally have a 61-second minute instead of smearing). Then someone never fails to freak out about the leap seconds because we have this oversimplified time abstraction that encourages you to ignore them. And instead of realizing they made a mistake in recommending UTC as the fundamental representation, BIPM is doubling down and is about to eliminate leap seconds entirely, so UTC will become just a worse version of TAI (because it will still have cause historical discontinuities with most software systems, but also drift away from solar time). I'm kinda pissed about it