Calculator Input
Example Data Table
| Date | ISO Week | US Sunday Week | Custom Rule | Use Case |
|---|---|---|---|---|
| 2026-01-01 | 1 | 0 | 1 | Release freeze boundary check |
| 2026-03-18 | 12 | 11 | 12 | Sprint planning snapshot |
| 2026-06-30 | 27 | 26 | 27 | Fiscal transition audit |
| 2026-12-31 | 53 | 52 | 53 | Year-end deployment validation |
Formula Used
The calculator applies several week-number rules because software teams often mix ISO calendars, local business calendars, and fiscal reporting windows.
- ISO week: Week 1 is the week containing January 4. The equivalent rule is the week containing the year’s first Thursday.
- US Sunday or Monday week: Week 1 starts on the first selected weekday of the year. Earlier dates become week 0.
- Custom week:
week = floor((date - firstWeekStart) / 7) + 1. The first week start depends on your chosen weekday and minimal first-week days. - Fiscal week: The calculator anchors a fiscal start month and day, then applies the same weekday and minimal-day logic.
- Sprint number:
sprint = ceil(selectedWeek / sprintLength)when the calculated week is greater than zero.
These rules make boundary testing easier for backlogs, reporting jobs, data imports, version tags, and date-driven automations.
How to Use This Calculator
- Enter the target date that needs validation.
- Select the timezone that matches your planning or reporting context.
- Choose ISO, US Sunday, US Monday, or Custom.
- When using custom rules, set the week start day and minimal first-week days.
- Set fiscal month and day if you report by fiscal periods.
- Enter sprint length to convert weeks into sprint numbers.
- Click the calculate button and review the summary above the form.
- Use the CSV or PDF buttons to save the output.
FAQs
1. Why can one date belong to another week-year?
Week-year systems group dates by full weeks, not only calendar years. Around New Year, a few days can belong to the previous or next week-year.
2. What makes ISO week numbering special?
ISO starts weeks on Monday and requires at least four days in the first week. This avoids inconsistent short opening weeks in many technical workflows.
3. Why do some US systems show week 0?
US Sunday and Monday styles can treat early January days as week 0 until the first full weekday-based week actually begins.
4. When should I use the custom mode?
Use custom mode when your product team, analytics pipeline, or internal dashboard uses a non-ISO week start or special first-week rule.
5. How is the fiscal week calculated here?
The calculator anchors your fiscal start month and day, then applies the selected custom weekday and minimal-days logic to build fiscal weeks.
6. Can I use this for sprint planning?
Yes. Enter your sprint length in weeks and the tool converts the chosen week number into a sprint bucket for planning.
7. Why is timezone included for a date-only tool?
Distributed teams often review the same milestone in different regions. Timezone selection keeps date interpretation consistent across shared planning workflows.
8. What does the Plotly graph help me verify?
It shows how week numbers move around the target date. That helps detect rollovers, fiscal resets, and sprint-boundary changes quickly.