The clock can turn back.
The timeline cannot.
Drag through a real clock change. The horizontal axis always moves forward. The local clock reading can jump backward or forward.
The orange dashed line is a change of labels, not elapsed time. Lord Howe Island shifts by 30 minutes, so a clock change does not always mean an hour.
A date, a time, a place.
Sometimes still not enough.
Try to book a moment. A backward jump repeats a range of clock readings; a forward jump skips a range. An ordinary reading matches exactly one moment.
Teaching snapshot: 2 January to 30 December 2026. Four places, with rules from IANA 2026e. A UTC offset identifies the occurrence; a place name alone may leave two choices.
Zero, one or two?
Six clock readings, including the exact edges of a skipped or repeated interval. How many real moments match each one?
Clock readings are names for moments. When the naming rule changes, some names are used twice and others are never used. To measure a duration, compare the moments on the same timeline.
Open the six-change source ledger
All 2026 changes in the three changing zones. Tokyo remains UTC +09:00. Each row shows two consecutive real minutes. Dates below the local readings are local dates; the UTC column is the exact change instant.
| Place | Last minute before | First minute after | Change instant, UTC |
|---|