Our sources

Every answer on UpcomingDays is worked out by the same date engine, and every rule it uses comes from a published source. This page lists them. If you think a date is wrong, please tell us.

Holidays

• US federal holidays and observed dates: 5 U.S.C. 6103, as applied by the US Office of Personnel Management.
• Easter and related days: the Gregorian (Western) church calculation of Easter Sunday.
• Jewish holidays: the Hebrew calendar, shown on the evening each holiday begins.
• Islamic holidays: the Umm al-Qura calendar of Saudi Arabia. Local moon sightings can move the actual day by one day, and our pages say so.
• Chinese New Year: published lunar calendar tables, cross-checked with the Hong Kong Observatory.
• Diwali: published Hindu calendar (panchang) dates for Lakshmi Puja.
• Tax Day: the IRS filing deadline rule (26 U.S.C. 7503): the next business day when April 15 is a weekend or a Washington, D.C. holiday.
• Public holidays in other countries: the open-source holidays project (MIT licence), which compiles each country’s statutory and regional holidays from official sources. We regenerate our data from it at least once a year; dates it marks as estimated (lunar holidays and days that wait for an official announcement) are labelled Estimated on our pages.
• School dates: district calendars vary, so we show a national midpoint and typical ranges by region.

Time and astronomy

• Time zones and daylight saving time: the IANA time zone database, which records every zone’s offsets and clock changes. US daylight saving rules come from 15 U.S.C. 260a.
• Week numbers: ISO 8601 (weeks start on Monday; week 1 contains the year’s first Thursday).
• Equinoxes, solstices and moon phases: computed with the open-source Astronomy Engine library, accurate to about a minute, dated in US Eastern Time.

Age and life tools

• Life expectancy: the US Social Security Administration period life table.
• Lifestyle effects: peer-reviewed studies, each cited with its DOI on the longevity and life expectancy pages.

How we check

The engine is tested against hand-checked reference dates for every holiday rule, week numbers, month arithmetic, daylight saving transitions and moon phases. Pages are recalculated every day, so countdowns never go stale. Read more in our editorial and corrections policy.