Crossing Clock

Key point

canal, strait and border crossings

Elapsed time is the difference between two instants, not between two clock readings. Subtracting the printed wall clocks gives the wrong answer whenever a DST transition falls between them.

A crossing — a canal transit, a strait passage, a border queue — is logged as two instants and read on the clock of whoever administers it. crossingTime gives the exact elapsed hours, with the entry and exit rendered in that one zone.

Each end gets an analog face, and the true elapsed duration sits above the naive wall-clock subtraction, flagged when the two disagree. A skipped or repeated hour is drawn on both faces as a wedge — dashed for an hour that never showed, doubled for one the clock ran through twice. On the fall-back night both faces read identically and are marked repeated, so identical clocks do not read as a bug.

1. Log the crossing

The clocks jump from 02:00 to 03:00 during this crossing, skipping an hour entirely. Seven hours actually pass; subtracting the wall clocks (00:00 to 08:00) says eight.

EntryExit

2. Elapsed, and what the clocks say

Entry

 

 

 

 

 

Exit

 

 

 

 

 

3. What crossingTime returns

 

Worth trying: the default preset already shows a skipped hour — New York’s clocks jump from 02:00 to 03:00, seven hours actually pass, and subtracting the wall clocks says eight. Switch to the added-hour preset to see the opposite: one hour actually passes, but both ends read 01:30, so subtracting the wall clocks says zero. Then the same spring-forward night read on a fixed −05:00 — a fixed offset observes no DST, so the wall-clock subtraction agrees and no wedge is drawn at all.

A day count for a stay is dwellTime, drawn in the Dwell Ledger.