Two categories of mistake that look like carelessness and are really ambiguity built into the units and the calendar.
Units that share a name and differ
| Unit | US | Imperial | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | 20% |
| Pint | 473 ml | 568 ml | 20% |
| Fluid ounce | 29.57 ml | 28.41 ml | 4% |
| Ton | 2,000 lb | 2,240 lb | 12% |
| Cup | 237 ml | 250 ml (metric) | 5% |
The fluid ounce is the cruel one, because it goes the other way. A US gallon is smaller than an imperial gallon, but a US fluid ounce is larger than an imperial one — the two systems divide differently, with 128 US fluid ounces to the US gallon and 160 imperial ones to the imperial gallon. Any assumption based on one comparison fails on the other.
Where this actually causes trouble
- Fuel economy. UK miles per gallon are about 20% higher than US figures for the same car, because the gallon is bigger. The car has not changed.
- Recipes. A US cup and a metric cup differ by 5%, which matters in baking and not in soup.
- Shipping weights. Short ton, long ton and tonne are three different things within about 12% of each other.
- Storage sizes. Drive manufacturers use 1 GB = 1,000,000,000 bytes; operating systems often display gibibytes at 1,073,741,824. A "1 TB" drive shows as roughly 931 GB, and nothing is wrong.
- Temperature. The rough conversion of doubling and adding 30 is wrong by several degrees across normal ranges. The actual formula is nine fifths plus 32.
The storage one generates a steady stream of complaints about missing capacity. It is a naming disagreement rather than a shortfall — the bytes are all there, counted in a different sized unit.
The expensive example
In 1999 NASA lost the Mars Climate Orbiter because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds.
The spacecraft approached Mars at the wrong altitude and was destroyed. Nothing was miscalculated; both numbers were correct in their own units, and nobody stated which units they were.
The lesson generalises to any shared spreadsheet. Put the unit in the column header. A column called "weight" is an accident waiting to happen; "weight_kg" is not. Unit Converter handles the arithmetic, but the discipline of labelling is what prevents the error in the first place.
Why time zones are harder than offsets
A time zone is not a number. "UTC+1" describes an offset at a moment; "Europe/London" describes a set of rules about which offset applies when.
London is UTC+0 in winter and UTC+1 in summer. Storing "UTC+1" records what was true on the day and becomes wrong at the next transition, which is why a recurring meeting drifts by an hour twice a year.
- Store UTC plus a named zone from the IANA database — America/New_York, Australia/Sydney.
- Never store a fixed offset for anything in the future.
- Transitions differ by country. The US and EU change on different dates, so for a few weeks each year the usual gap between them is an hour out.
- The southern hemisphere is reversed, so a UK–Sydney gap ranges from 8 to 11 hours across the year.
- Some zones are not whole hours — India is +5:30, Nepal +5:45, parts of Australia +8:45.
- Some places have no daylight saving at all, including most of Arizona, and several countries have abolished it recently.
The mismatched transition dates catch international teams every spring and autumn. A meeting that has run at a comfortable time for months suddenly lands an hour earlier for one side, and nobody changed anything.
Scheduling across zones
- Send a calendar invitation rather than a time in an email. The invitation carries the zone and every attendee sees it correctly.
- If you must write a time, name the zone — and use the city, not an abbreviation. "CST" is ambiguous between North America and China.
- Check the date as well as the hour. A 9am Tuesday call in London is Monday evening in Los Angeles.
- Find the genuine overlap before proposing. Beyond about eight hours apart, someone is outside working hours whatever you choose.
- Rotate the inconvenience for recurring meetings across distant zones.
- Avoid the fortnight around transitions for anything critical, or verify it explicitly.
Timezone Converter shows the same moment across several zones at once, which is the view you want when proposing a slot. Abbreviations are worth avoiding entirely — besides CST, "IST" covers India, Ireland and Israel.
Frequently asked questions
How much do US and imperial gallons differ?
About 20% — a US gallon is 3.785 litres and an imperial gallon 4.546 litres. Pints differ by the same proportion. Confusingly, fluid ounces go the other way: a US fluid ounce is slightly larger than an imperial one, because the two systems divide the gallon differently.
Why is UK fuel economy higher than US for the same car?
Because the imperial gallon is about 20% larger, so the same efficiency yields a higher miles-per-gallon figure. The car is identical; only the unit changed.
Why does my 1 TB drive show as 931 GB?
Manufacturers count 1 TB as a trillion bytes, while operating systems often display gibibytes of 1,073,741,824 bytes each. It is a naming disagreement rather than missing capacity — all the bytes are there.
What is the correct Celsius to Fahrenheit formula?
Multiply by nine fifths and add 32. The common shortcut of doubling and adding 30 is off by several degrees across normal temperatures.
Why do my recurring meetings shift by an hour?
Because a fixed offset was stored rather than a named time zone. Offsets change with daylight saving, and the US and EU transition on different dates, so for a few weeks each year the gap between them differs from usual.
How should I store times for future events?
UTC plus a named IANA zone such as Europe/London or America/New_York. A fixed offset records what was true when you saved it and becomes wrong at the next transition.
What is the best way to schedule across time zones?
Send a calendar invitation rather than writing a time, since the invitation carries the zone. If you must write one, name the city rather than an abbreviation — CST is ambiguous between North America and China, and IST covers India, Ireland and Israel.