Day of the Week Calculator
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What Your Result Means
- Day name: The weekday (Monday through Sunday) that your chosen date falls on, useful for planning events or verifying historical dates.
- Day of year: Where the date sits in its calendar year, from 1 (January 1) to 365 or 366 (December 31). Helpful for ordinal-date systems and project timelines.
- ISO week number: The week of the year according to ISO 8601, where weeks start on Monday and the first week contains the year's first Thursday.
- Days from today: How far in the past or future the date is relative to right now, updated in real time.
How This Calculator Works
You enter any date. The tool creates a JavaScript Date object set to midnight local time and reads the built-in getDay() index to determine the weekday. Day-of-year is calculated by counting days elapsed since January 1. The ISO week number follows the ISO 8601 rule where week 1 is the week containing the first Thursday of January. Days from today is a simple timestamp subtraction divided by 86,400,000 milliseconds per day.
Quick Questions
How far back in history can I look up a date?
JavaScript's Date object supports years from roughly 271,821 BCE to 275,760 CE. In practice, any date in the common era works reliably. Very ancient dates may not align with the historical Julian calendar.
What is an ISO week number?
ISO 8601 defines week 1 as the week containing the year's first Thursday, with weeks starting on Monday. This means January 1 can sometimes fall in week 52 or 53 of the previous year.
Does this account for the Julian-to-Gregorian calendar switch?
No. The calculator uses the proleptic Gregorian calendar throughout. Historical dates before the Gregorian adoption (1582 in most of Europe) may differ from contemporary records that used the Julian calendar.
Why does "days from today" update when I reload?
The calculation compares your chosen date to the current device time. Each page load recalculates the difference, so it always shows the up-to-date distance from right now.
Sources
- Wikipedia — ISO 8601 (week numbering rules and date format standards)
- MDN Web Docs — Date.getDay() (JavaScript weekday index reference)
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.