How time zones and daylight saving are handled
Every local time on this site — the world clock, the sunrise times, the tide times — is produced the same way, and it is deliberately not the obvious way.
No offsets are stored. Anywhere.
The tempting design is a table of cities and their UTC offsets. It is also wrong within about a year, because governments change daylight saving rules with little notice and every stored offset becomes a small lie until someone rebuilds the site.
What is stored instead is the IANA time zone name — America/Halifax, Asia/Kolkata, Australia/Lord_Howe. The offset is then asked for at the moment it is needed, from your own device's internationalisation engine, which is backed by the IANA database and updated by your operating system.
The practical consequence: when a country moves its clocks, these pages follow as soon as your device does, with no rebuild here at all.
Detecting daylight saving
There is no flag in the data saying "this zone uses DST". It is worked out, with a rule short enough to state completely:
1. Ask for the zone's offset on 15 January and on 15 July of the current year.
2. If the two are equal, the zone does not observe daylight saving. Say so and stop.
3. If they differ, the one further from UTC is the daylight-saving offset.
4. Compare today's offset against that one to say whether the zone is on daylight time right now.
Two dates, six months apart, because a rule that sampled only summer would call the southern hemisphere backwards. Australia's daylight saving is in January; the comparison above handles it without a special case, since it never assumes which half of the year is which.
Which page a zone links to
Around 2,300 place pages carry a one-line "it's 5:35 PM in Portland right now" strip, and each has to link somewhere sensible on the world clock. The resolution runs in three steps, and the link text changes with it — which is the point.
This city has a world clock page. The link names the city.
It does not, but its zone has a representative page. The link names the zone: "Pacific Daylight Time on the world clock". Portland has no page of its own and lands on Los Angeles, so promising "Portland on the world clock" would be a promise the destination cannot keep.
Neither. The link goes to the hub and says so.
Zones are matched first by exact IANA name, then by the daylight-saving-independent name the browser gives them — which is how America/Detroit finds the Eastern Time page without anyone maintaining a list of aliases.
The awkward cases, and what happens
One genuine limitation: the January-versus-July comparison describes the current year. A zone that abolishes daylight saving partway through a year is described correctly for the year as a whole but can be briefly misleading about the transition itself.