Gregorian Calendar Calculator
Pick a date to calculate weekday, ISO week number, day of year, leap-year status, and more.
The Gregorian calendar is the world standard for civil dates, and understanding how it works can help with planning, reporting, scheduling, and historical analysis. This calculator gives you practical date metrics in one click.
What this Gregorian calendar calculator does
This tool evaluates a selected date and returns:
- Day of week (Sunday through Saturday)
- Leap year status according to Gregorian rules
- Day of year (1 to 365 or 366)
- ISO week number and ISO week-year
- Quarter of year (Q1 to Q4)
- Days in month and days remaining in year
- Date difference if a comparison date is entered
How to use it
1) Choose a date
Use the first input to select the date you want to analyze. You can also click Use Today for a quick result.
2) Optional: compare two dates
If you fill in the second input, the calculator also shows the signed day difference between the two dates.
3) Click Calculate
The result panel updates instantly and includes a month-view calendar with your chosen day highlighted.
Gregorian leap year rule (quick reference)
The Gregorian system corrects drift in the solar year using this logic:
- A year is a leap year if it is divisible by 4,
- except years divisible by 100 are not leap years,
- except years divisible by 400 are leap years.
So 2000 was a leap year, but 1900 was not.
Why ISO week numbers matter
Many organizations use ISO week numbering for operations, payroll, logistics, and project tracking. In ISO 8601:
- Weeks begin on Monday.
- Week 1 is the week containing the first Thursday of the year.
- Some dates near January 1 may belong to the previous ISO year, and vice versa near December 31.
Common use cases
- Academic calendars and term planning
- Financial reporting by week/quarter
- Deadline counting and contract timing
- Historical date comparisons
- Software testing for date-based logic
Frequently asked questions
Is this calculator using the Gregorian calendar only?
Yes. All calculations are based on Gregorian calendar rules.
Does this tool account for time zones?
The calculator treats each selected value as a calendar date and computes results at date level (not time-of-day), which avoids most timezone confusion for day-based calculations.
Can I calculate historical dates?
Yes. You can enter any valid date supported by your browser date picker and get instant metrics.