Enter a start date and whole years, months, weeks or days. Each unit has its own increment: years and months offer 1, 3, 5 and 10; weeks offer 1, 5, 10 and 20; days offer 1, 10, 30 and 90. On mobile, plus is above the number and minus below it. Swipe the one-number wheel to choose an increment, or focus it and use Up/Down, Home or End. Reset restores increment 1 for every unit.
Both results are visible together. Subtraction is on the left and addition on the right. Each result has a date-range link and Copy. Only addition has Share and a Countdown link. The Countdown link or switching directly from Add/Subtract to the Countdown tab starts a timer to midnight at the beginning of the latest Add result date in the selected site timezone. The selected date format stays the same when switching tabs. Day-of-year and week-number details also align to the corresponding edge. Language is chosen manually. Unlocalized URLs use English; localized URLs keep their language. Changing language or using a calculator menu preserves the selected country, region, timezone, date format, IP/Manual mode and theme. On the home page, calculator menus switch tabs in place without repeating IP detection. The menu closes when you choose a tool, click elsewhere or press Escape. Between pages, preferences are transferred once through temporary browser storage; if it is blocked, some settings return to defaults and detection may repeat.
Week numbers use ISO 8601: weeks start on Monday, and week 1 contains January 4. Dates near New Year can belong to the previous or next ISO week-year. A week number is a calendar detail, not a count of working weeks.
Worked example
Start on October 12, 2026 and enter 10 days. The left result is October 2, ISO week 40; the right result is October 22, ISO week 43. With a Days increment of 30, plus changes 10 to 40 without changing the other units.
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