Time and place

Local time, UTC and timezones

Goal: understand why "9pm" is not a moment in time, and what to use instead.

Observe

Ask someone in another country what time it is. They will give you a different number than you would.

Neither of you is wrong. You are both reporting the same instant, in different units.

Predict

Before running anything, answer these:

1. If it is 21:00 where you are, is it 21:00 everywhere?
2. If you and a friend on another continent both look at the Moon "at 9pm",

are you looking at the same time?

1. How many distinct moments does "3am on the last Sunday in October" describe

in a country that uses daylight saving?

Write your answers down.

Learn

A bare time is not an instant

"21:00" is incomplete information. It is like saying "it is 15 degrees" without saying Celsius or Fahrenheit, a number with the units left off.

To specify a real moment you need a time and a reference frame:

• 21:00 BST: a specific instant
• 21:00 UTC: a different specific instant
• 21:00: not an instant at all

The Moon moves about half a degree (its own width) every hour. An ambiguous time is an ambiguous sky.

UTC: the shared reference

Coordinated Universal Time is the reference everyone measures against. It does not change for daylight saving. It is the same everywhere on the planet simultaneously. Every timezone is defined as an offset from it.

Note that offsets are not all whole hours. India is +5:30, Nepal is +5:45, parts of Australia are +8:45. Any code that assumes whole-hour offsets is broken for well over a billion people.

Why time zones exist at all

Before railways, every town kept its own solar time and nobody minded. Trains made that impossible; a timetable needs one clock. Standard time zones were the fix. Why UTC exists tells the whole story; it is genuinely a good one.

Daylight saving time

Many countries shift their clocks seasonally. This creates two genuinely horrible situations twice a year:

• Spring forward: clocks jump 02:00 → 03:00. The times between never

happened. 02:30 on that date does not exist.

• Autumn back: clocks jump 03:00 → 02:00. The times between happen

twice. 02:30 on that date is ambiguous; there are two of them, an hour apart.

This is why software that stores local timestamps has a bad October. UTC never does either of these things.

Southern hemisphere countries shift in the opposite months. Many countries do not shift at all. Some have changed policy recently. Do not hard-code any of it; that is what the timezone database is for.

The timezone database

Timezone rules are political and they change. Governments alter offsets, adopt or abandon DST, and occasionally move a whole country across the date line.

The IANA timezone database records all of it, historically. Names look like:

Europe/Lisbon
America/Argentina/Cordoba
Africa/Nairobi
Asia/Kathmandu
Pacific/Auckland

The format is Region/City, a city rather than a country, because countries sometimes span several zones or change their minds. Python reads this database through zoneinfo, which Moonfield uses.

Avoid abbreviations like EST or CST. They are ambiguous, CST is used for US Central, China Standard, and Cuba Standard time, which are hours apart.

Run

moonfield doctor

Look at the time section:

  - System timezone: Europe/London
  - Local time: 2026-08-16 22:00:00 BST
  - UTC time:   2026-08-16 21:00:00 UTC
  - Offset from UTC: +1.00 hours

Now watch the same instant expressed two ways:

moonfield phase --date "2026-08-16T21:00" --timezone UTC
moonfield phase --date "2026-08-16T17:00" --timezone America/New_York

Same instant. Same Moon. Different local clocks.

Change one variable

Run the same local wall-clock time in different zones:

moonfield phase --date "2026-08-16T21:00" --timezone Europe/London
moonfield phase --date "2026-08-16T21:00" --timezone Asia/Tokyo

These are different instants (nine hours apart), so the Moon has moved. Check the illumination percentages: they differ slightly.

This is the whole lesson in two commands. "9pm" is not one moment.

Validate

Pick any online world clock and compare it against moonfield doctor. If your local time or offset is wrong, fix it before continuing; everything downstream inherits the error.

Explain

Why does Moonfield compute in UTC rather than your local time?

Because UTC is monotonic and unambiguous. It never repeats an hour, never skips one, and never depends on politics. Local time does all three.

Why display local time at all, then? Because "the Moon rises at 20:47 UTC" is useless to a person standing in a garden in Auckland. The computation needs rigour; the human needs relevance. You do both, and you label them.

Checkpoint

• I can explain why "21:00" is not a specific moment
• I know what UTC is and why it does not observe daylight saving
• I know some offsets are not whole hours
• I can name the two things DST does to local time twice a year
• I know why IANA names use cities rather than countries
• I understand the convention: compute in UTC, display local

Try it yourself

1. Find your IANA timezone name. Is it the city you expected?
2. Run moonfield phase for the same UTC instant expressed in three zones.

Confirm the Moon is identical.

1. Run it for the same local time in three zones. Confirm the Moon differs.
2. Find a country with a 45-minute offset. Why might a government choose that?
3. Look up when your country last changed its DST rules.

Questions to think about

• If everyone used UTC and abandoned local time, what would actually break?
• Why do timezone boundaries follow political borders rather than meridians?
• China spans about 60 degrees of longitude and uses one timezone. What is

solar noon like in its far west?

• Is there any moment when the whole world is on the same calendar date?

Common questions

Is UTC the same as GMT? Close enough for this curriculum, and different in principle. GMT is based on Earth's rotation; UTC is atomic time kept within 0.9 seconds of it. They agree to under a second.

Why "UTC" and not "CUT" or "TUC"? A compromise between English "Coordinated Universal Time" and French "Temps Universel Coordonné". Neither language got its acronym.

What is Zulu time? UTC. Military and aviation phonetic alphabet, zone Z is offset zero. The Z on the end of 2026-08-16T21:00:00Z means exactly this.

Getting stuck?

Getting Unstuck · Discussions

Go deeper

• Why UTC exists
• Julian Day: what astronomers compute with instead
• Read src/moonfield/time.py

Next: Julian Day.