Calendar dates and week numbers
Standard date views use the Gregorian calendar. ISO week numbers use Monday as the first day of the week; week 1 is the week containing the year's first Thursday. A date near New Year can belong to a different ISO week-year than its calendar year.
A Sunday-first month grid changes the layout, not the ISO definition. Current-day displays can also depend on the browser timezone, the server-generated initial view, and the selected location. Include your timezone when reporting a date discrepancy.
Holiday coverage
The current US holiday set is calculated from fixed dates and weekday rules. The Canadian set is a selected list, not a complete list for every province, territory, or employment category. Weekend substitute holidays and employer-specific closures are not fully represented.
Business-day totals currently count Monday to Friday and subtract listed US holiday dates. They may differ from an employer's working calendar, especially where an observed holiday moves to a weekday. Confirm official dates with the relevant government or employer before relying on a total.
Moon phases and lunar dates
The phase model measures elapsed time from a reference new moon on January 6, 2000 at 18:14 UTC, using a mean cycle of 29.53058867 days. Illumination is approximated with a cosine curve, and phase labels group positions within that cycle.
The real lunar cycle varies. These views illustrate a rhythm rather than calculating precise astronomical event times. A phase date can differ from an observatory's published date or a local civil date. Moonrise and moonset views are particularly simplified: they do not model the observer's horizon or a full lunar orbit.
Sunlight and seasons
Sunlight duration currently uses a simplified seasonal curve influenced by latitude. When longitude and timezone are available, an estimated solar-noon adjustment accounts for the timezone offset and an equation-of-time approximation.
This is not a precision sunrise calculator. Terrain, elevation, atmospheric conditions, and high-latitude polar day or night are not adequately modeled. Do not use it for navigation or safety-critical planning. Season progress uses Northern Hemisphere meteorological seasons; astronomical season-marker pages contain approximate calendar dates.
Personal and creative calendars
Life in Weeks compares a chosen birthday with the current date and draws 52 cells per displayed year. Its current display is capped at 4,160 weeks. Calendar-week boundaries and leap days mean the view is an illustration, not an exact count of seven-day periods since birth.
Daily colors are generated deterministically from the date; they are creative selections, not measurements or predictions. Daily quotes are a rotation of original prompts. User-created birthday, event, and activity calendars depend on the information their creators enter.
Sources and corrections
Reference sources are linked below for comparison. Naming a source does not mean that every page is synchronized with it in real time. Famous-birthday pages use Wikidata with a bundled fallback; external records can contain gaps or errors.
If an answer conflicts with a reliable source, send the date, page URL, region, timezone, and source using our corrections guide. This page documents the current implementation and its limits rather than claiming a precision the tools do not provide.