Date standard
All calculations use the proleptic Gregorian calendar — the modern civil calendar extended uniformly forward and backward in time, via the standard JavaScript date implementation (date-fns).
A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So February 29 exists in 2024 (divisible by 4) and in 2000 (divisible by 400), but not in 2100 (a century year not divisible by 400). Adding one day to February 28, 2024 lands on February 29, 2024.
Timezone baseline
Relative-date pages ("30 days from today", countdowns) are generated against the UTC calendar day: at build time, "today" is the current day in UTC. The site is rebuilt daily shortly after midnight UTC so the static answers stay aligned with the current UTC day.
In your browser, each answer is then re-checked against your local calendar day. When your local day differs from the UTC build day, the answer is recomputed for your timezone and the label changes to "Updated for your timezone".
Every relative-date answer carries a "Calculated as of <date> (UTC)" label showing the reference day used at build time.
Counting conventions
- Elapsed vs inclusive days. "Days between" reports elapsed days by default: January 1 to January 31 is 30 elapsed days. The inclusive count adds both endpoints, so the same span is 31 days including the start date.
- Adding days excludes the start date. The count starts the day after your start date, so 1 day from July 29 is July 30. An optional "Count the start date as day 1" toggle counts the start date itself and shifts the result one day toward the start.
- Weeks are exactly 7 days. 4 weeks from today is exactly 28 days from today.
- Months are calendar months. Adding a month moves to the same day of the target month, with a month-end caveat: when that day doesn't exist, the result clamps to the last day of the month — 1 month from January 31 lands on the last day of February (February 28, or February 29 in a leap year).
Business days
A business day is any day that is not a weekend day — the weekend is Saturday and Sunday — and not an excluded date.
You can optionally exclude the 11 US federal holidays. Holidays are matched on their observed date: when a fixed-date holiday falls on a Saturday it is observed the preceding Friday; when it falls on a Sunday it is observed the following Monday. Holiday data follows the OPM federal schedule and was last reviewed 2026-07.
Custom exclusion dates (company shutdowns, personal days off) are supported in the business days calculator. Holiday calendars for other countries are not currently offered — only the US federal schedule.
Holiday dates
Fixed-date holidays use the same month and day every year (Christmas is always December 25). Movable holidays follow explicit rules:
- Easter is computed with the Anonymous Gregorian algorithm — the first Sunday after the first full moon on or after the spring equinox, always between March 22 and April 25.
- Nth-weekday rules pick a specific occurrence of a weekday in a month — Thanksgiving is the fourth Thursday of November, Father's Day is the third Sunday of June, Memorial Day is the last Monday of May.
Each holiday page labels where the holiday is primarily observed (for example "United States" or "International"). Season start dates are Northern Hemisphere approximations — the exact astronomical equinox or solstice can vary by a day or two.
Accuracy & corrections
Spot a wrong answer or an outdated holiday date? We want to know. Email [email protected] with the page URL and the result you expected, and we'll verify it against the rules above and fix it.
Put it to work
Try the calculators, browse the holiday countdowns, or embed a countdown on your own site with our free embed widgets.