Methodology
This site publishes a lot of computed numbers — sunrise in one city, the next high tide at one station, what time it is somewhere else. These pages say how each one is worked out, and where it stops being reliable.
The pages
How sunrise and sunset are calculated
The solar-position method, why the horizon is −0.833°, what is not modelled, and a measured bound on the error.
How the moon phase is calculated
Meeus's periodic-term series for position, illumination and the exact instant of a full moon — and where the truncation shows.
Where the tide predictions come from
NOAA CO-OPS, what MLLW heights mean, and the weather effects an astronomical prediction deliberately excludes.
How time zones and daylight saving are handled
Why no UTC offset is stored anywhere, and how daylight saving is detected from two dates six months apart.
How accurate is a browser timer?
Measured setInterval drift, tick error under CPU load, and the three behaviours that could not be measured.
Two rules these pages follow
Every accuracy figure was measured here. The two pages that quote one read it from a file written by a measurement script in the repository, and the build fails outright if that file is missing or incomplete. A methodology page that invented its own error bar would be worse than no methodology page, so it is not possible to ship one.
What could not be measured is named, not estimated. The browser timing page lists three behaviours it could not reproduce, with the reason for each, instead of quoting numbers from elsewhere. One test was written, run, and discarded when it turned out to be measuring nothing.
What none of this is for
These are consumer tools. The astronomy is good enough to plan an evening walk or a photograph; the tide predictions are NOAA's own, and NOAA excludes weather from them. Nothing here is suitable for navigation, aviation, safety-critical timing, scientific measurement, or any decision where being a few minutes out has consequences. Where that matters most — tides — the page says so at length.