Day of the Week Calculator
Find which weekday any date in history or the future falls on.
About Day of the Week Calculator
The weekday is determined by parsing the date as a UTC midnight timestamp and reading its getUTCDay() value, which avoids timezone-induced off-by-one errors. The ISO 8601 week number is computed by finding the Thursday of the week the date belongs to — ISO weeks always start on Monday and week 1 is the week that contains January 4 (equivalently, the first Thursday of the year). The day-of-year is the ordinal count from January 1.
Formulas
ISO Week 1 rule
Week 1 is the calendar week containing January 4th (or equivalently, the first Thursday of the year).
Day of year
Day of year = (date timestamp − Jan 0 timestamp) ÷ ms-per-day
Worked example
With the default values below, Day of the Week Calculator returns Full Date of Friday, July 4, 2025. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Date
- 2025-07-04
- Show ISO Week & Day-of-Year
- Yes
Results
- Full Date
- Friday, July 4, 2025
- Day of the Week
- Friday
- Weekday or Weekend
- Weekday
- ISO Week NumberISO 8601 week, where week 1 contains the first Thursday of the year.
- 27
- Day of YearOrdinal day number within the calendar year (1 = Jan 1).
- 185
- Days Left in Year
- 180
- Leap Year
- No
- Next Same Weekday
- 2025-07-11
Step by step
- 1Parsed date (UTC)= 2025-07-04
- 2Day of week index (0 = Sun)= 5 → Friday
- 3Day of year= Day 185 of 365
- 4ISO week= W27
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is an ISO week number?
ISO 8601 weeks run Monday to Sunday and are numbered 1–53. Week 1 is the first week that contains a Thursday, which means January 1 can be in week 52 or 53 of the previous year if it falls late in the week.
Can I look up historical dates before 1970?
Yes. JavaScript's Date object handles the proleptic Gregorian calendar back to at least 100 AD for day-of-week calculations.
What does 'day of year' mean?
It's the ordinal position of the date within the calendar year. January 1 = day 1, December 31 = day 365 (or 366 in a leap year).
Why does the ISO year sometimes differ from the calendar year?
The last few days of December and the first few days of January can belong to the adjacent ISO year. For example, December 31, 2018 was in ISO week 1 of 2019.