Calendar Tools
Handy calendar utilities to look up historical dates, find out which day of the week a date falls on, and verify ISO standard week numbers.
Available Calculators (2)
Why Use Our Calendar Tools?
Calculations execute locally inside your web browser. No server round-trips, ensuring instant responsiveness.
We never record, save, or transmit any dates or personal information you enter into the tools.
Calendar-pure algorithms protect your results from subtle daylight saving distortions and leap year bugs.
Category Questions & Answers
What is an ISO 8601 week number?
ISO 8601 defines Monday as the first day of the week. Week 1 of any year is the week containing the first Thursday of that year (or containing January 4th).
Can a date in early January belong to the previous year’s week?
Yes! For instance, if January 1 is a Friday, Saturday, or Sunday, it belongs to the final week (Week 52 or 53) of the preceding year under ISO 8601.