A simple, free week calculator to find week numbers for any date, determine dates from week numbers, and calculate weeks between dates. Perfect for planning and scheduling by weeks. No Signup Required.
The concept of a seven-day week dates back to ancient Babylon around 600 BCE, but became standardized across the Western world only after Emperor Constantine adopted it in 321 CE. Interestingly, the ISO week numbering system can lead to counterintuitive results—December 31 sometimes falls in week 1 of the following year, while January 1 can be in the last week of the previous year. The business world often uses "4-4-5 calendars" where each quarter contains two 4-week months and one 5-week month, totaling 13 weeks per quarter. This system helps standardize financial reporting by ensuring comparable periods. Some cultures historically used different week lengths: ancient Romans initially used 8-day weeks, while the French Revolutionary Calendar briefly implemented 10-day weeks in the late 18th century.
Week calculation algorithms must handle numerous edge cases, particularly around year boundaries. The ISO 8601 standard defines the first week of the year (W01) as the week containing the first Thursday, which means complex calculations involving the day of week of January 1st. Implementing week calculations requires modular arithmetic to handle cyclical patterns and careful attention to leap years. Modern date libraries use specialized algorithms like Zeller's Congruence or variations of Gaussian algorithms to efficiently determine the day of week for any date. When calculating weeks between dates, developers must decide whether to count partial weeks or only complete weeks, and whether to include or exclude the boundary dates—choices that can significantly affect results. Timezone considerations add another layer of complexity, as the same timestamp can represent different dates in different regions.
Our Week Calculator allows you to find the week number for any date, determine which dates fall in a specific week number, and calculate weeks between dates. It's perfect for project planning, scheduling, and understanding week-based timelines.
Simply enter your date in the calculator, and it will automatically display the corresponding week number according to ISO standards (where week 1 is the first week with at least 4 days in January).
Yes! Enter the week number and year, and the calculator will show you the start date (Monday) and end date (Sunday) of that week, making it easy to plan week-based activities.
Yes, our week calculator uses the ISO 8601 standard for week numbering, where weeks start on Monday and the first week of the year is the one containing the first Thursday of the year.
Yes, the calculator can determine the number of complete weeks between any two dates you specify, which is useful for project timelines and planning.
Yes, we take data security seriously. Your date information is processed locally in your browser and never sent to our servers. This ensures complete privacy and security of your information.
The calculator can show both complete weeks and partial days when calculating between dates, giving you precise information for your planning needs.