A map of world time zones shows lines that wander far from the meridians they nominally follow. Every deviation records a decision made by a government rather than by the sun.
The original problem was railways
Before standardized time, towns kept local solar time, and noon differed by minutes between neighboring places. That was tolerable until scheduled transportation made it dangerous.
Railroads needed a single reference to publish timetables and avoid collisions, and standard time zones were adopted to serve that operational need.
The astronomical framework of fifteen-degree bands provided a starting structure, but adoption depended on legislatures and railway companies rather than on astronomers.
Borders override meridians
Splitting a country or a city across two zones creates constant administrative friction, so boundaries are routinely bent to keep political units whole.
Some large countries observe a single time across their full width, accepting that solar noon arrives at wildly different clock times in different regions.
Others split into several zones, and the internal boundaries often follow state, provincial or county lines rather than any geographic logic.
Commerce pulls zones toward partners
Sharing business hours with a major trading partner has real value, since overlapping working days shorten decision cycles and reduce the cost of coordination.
Territories have shifted zones for exactly this reason, moving to align with the markets or capitals they deal with most.
Offsets that are not whole hours exist for similar reasons, representing compromises between solar reality and the desire to sit closer to a neighbor's clock.
Daylight saving adds a seasonal layer
Where seasonal clock changes are observed, the effective offset shifts part of the year, and the transition dates differ between countries and hemispheres.
Some jurisdictions within a country opt out entirely, which means the relationship between two places can change several times a year.
Scheduling across regions therefore requires knowing both zones and both sets of transition rules, which is why calendar software handles this so imperfectly.
Maintaining the rules is ongoing work
Governments change their time rules with varying notice, and computer systems worldwide depend on a shared database of those rules to convert times correctly.
Updates propagate through operating systems and applications, and devices that have not received an update apply outdated rules after a change takes effect.
Which is why a phone occasionally shows the wrong time after a policy shift. The device is correctly applying a rule that is no longer in force.