What this calculation means
A calendar date carries more context than its weekday. This calculator also returns the ISO week-year/week number and ordinal day of the year so the answer is useful for schedules, records and technical date checks.
How to use it
Step 1
Enter the calendar date in the date field.
Step 2
Check the weekday and ISO week-year/week number.
Step 3
Use day-of-year and leap-year context when the date is part of a schedule or record.
Useful for
- • Checking historical or future weekdays
- • ISO-week schedule references
- • Day-of-year reporting and logs
Limits to keep visible
- • This uses the proleptic Gregorian calendar supported by JavaScript dates. Historical jurisdictions adopted the Gregorian calendar at different times.
- • A calendar date alone has no time zone or time-of-day context.
Frequently asked questions
What is an ISO week?
ISO 8601 weeks begin on Monday. Week 1 is the week that contains January 4, so late-December dates can belong to ISO week 1 of the following year.
What is day of year?
It is the ordinal position of a date within its year, with January 1 as day 1.
Does the weekday depend on time zone?
A pure civil date does not. A timestamp near midnight can map to different dates in different zones, but this calculator works from the date you enter.