A world map of time zones shows boundaries that wander, double back and occasionally swallow whole countries. The shapes come from politics and commerce, not from the position of the sun.
The clean version exists only in theory
The tidy scheme divides the globe into twenty-four bands of fifteen degrees each, one for every hour of the earth's rotation.
That scheme fixes a real problem. Before it, towns kept local solar time, and a railway timetable crossing a region had to reconcile dozens of slightly different clocks.
What the scheme cannot do is tell a government where to put the line, and the answer has always been wherever it is convenient.
Boundaries follow jurisdictions
Splitting a city, a province or a small country between two zones creates daily friction for schools, courts, transport and broadcasting.
So lines are drawn along administrative borders instead, which is why the boundary steps sideways rather than running straight down a meridian.
Large countries make the opposite choice in the other direction, keeping a single national time across a span that geometry says should hold several zones.
Trade pulls clocks towards partners
A country whose banking, shipping and exports point in one direction gains from sharing a working day with that direction.
Shifting an hour can add overlap with a major partner's business hours, and the cost of being out of step is measured in unreturned calls and missed settlement windows.
Changes of this kind are usually announced as economic decisions, and they are one reason a zone can move without any change to geography.
Some offsets are not whole hours
Several places keep clocks offset by thirty or forty-five minutes rather than a full hour.
These arise where a country sits between two bands and neither fits its solar noon well, so it compromises rather than pushing sunrise across the working day.
The half-hour offsets are the clearest evidence that the system is negotiated rather than derived, since nothing astronomical requires them.
Seasonal clock changes add another layer
Daylight saving shifts a zone by an hour for part of the year, and adoption is uneven between neighbouring countries.
The gap between two places therefore changes several times a year, and for a few weeks it can differ from the value people have memorised.
Software handles this with a maintained database of rules rather than arithmetic, because there is no formula for a decision that a legislature can revise at any time.