Calendar: weekday, week number & days between dates
Weekday, ISO week, day of the year, leap year, days between two dates, and a date plus a number of days.
Weekday
Thursday
Day of the year
1
ISO week
1 / 2026
Leap year?
No
Days from first to second date
357
In weeks and days
51 wk 0 d
Weekday of the second date
Thursday
days
Resulting date
2026-04-11, Saturday
Gregorian calendar. ISO 8601 weeks start on Monday; week 1 is the week containing the first Thursday of the year, so early January can belong to the previous year's last week.
How it works
The Gregorian calendar repeats every 400 years. ISO 8601 numbers weeks from the one that contains the year's first Thursday.
Formulas
- Leap year: divisible by 4, except centuries not divisible by 400
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