I have a complicated relationship with time, and part of the reason is because time is a more complicated thing than it appears.

I’m late, I’m late

One of the neat things about my particular biology is that I have both a disrupted sleep schedule and time blindness.

My natural sleep cycle is, I suspect, not your average morning-to-night sleep schedule that we’ve built modern civilization around. I say “I suspect” because I&ldsquo;m not sure I’ve ever actually been able to be on my natural sleep cycle. A thing about living in a society that is built around expectations on when people are asleep and when people are awake is that if those people want to not fail out of school, hold a job, go to doctor appointments, have a social life, be able to run errands, or interact with other people, they kinda need to be awake during the hours people are expected to be awake. Whether that’s aligned with their natural sleep cycle or not. And given that I haven’t (yet!) had a sustained period of time where I was not subject to those constraints, I don’t actually know what my natural sleep cycle looks like. Instead I run on the “battery-always-on-empty” sleep cycle, where I fall asleep when my body can’t stay awake anymore and wake up when it has enough energy for a day again or when external stimuli force me to (a fancy way of saying “my husband shakes me awake and hands me caffeine before I can fall asleep again”).

Talk about time blindness

But what is “late”, anyways?

A mental model of time

Talk about half-lives/atomic clocks, footnote about gravitational waves and dark matter discovery re: time moving slower.

Talk about seconds, etc. not existing, just being labels we’ve given to some amount of time. Talk about it being based on the sun. Talk about lunar calendars.

Time is an ugly hack

Talk about how time is trying to marry a physics-based system to a human system.

Talk about leap years (because the time it takes the earth to orbit the sun isn’t evenly divisible by the time it takes the earth to rotate on its axis). Days are different lengths depending on where in orbit we are around the sun, but still important for circadian rhythms. If we defined a day based on the time it takes to rotate the earth, every 15 years or so day and night would flip. Oops. But years are important for understanding climate, so if we define a year based on days, the seasons would jump around months. Oops. Talk about fractions being hard, so we just use fuzzy numbers with an exception every so often and it’s Good Enough.

Talk about how the earth slows down and speeds up, so we have leap seconds that have turned into smear seconds.

Talk about daylight savings time and how it’s political/legal, meaning what time it is depends on not only your timezone but your municipality within that timezone. Talk about places observing/not observing daylight savings time.

Advice for software engineers

Practical advice for people who need to work with time.

Don’t rely on time

Talk about clock skew

UTC is your friend

Talk about how daylight savings time doesn’t apply to UTC and it’s standard which makes it ideal to coordinate on

One timezone is better than two

Standardize on a timezone for internal systems. If you don’t have any business needs around it, use UTC. If you do have business needs, pick the one that’s aligned with business needs. Talk about how using multiple timezones exposes all these edge cases for people to remember and think about and for systems to account for.

Don’t convert timezones by hand

Talk about how timezones are ever-shifting and converting between two is complicated, and people should let the timezone database paper over that for them.