📅 Date Calculator
Add or subtract time from a date, count the days between two dates, or look up a date's weekday and week number.
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How the Date Calculator Works
This tool performs calendar-accurate date arithmetic across four modes: adding or subtracting time from a date, counting the days between two dates, finding which day of the week a date falls on, and finding a date's ISO week number. Need to count only weekdays, or add business days while skipping weekends? Use the Business Days Calculator instead.
Method
Count Days: difference = |dateB − dateA| in calendar days
Week Number: ISO 8601 — week 1 is the week containing the year's first Thursday
Worked examples: Adding 30 days to January 1, 2026 lands on January 31, 2026. From January 1 to December 31, 2026 is 364 days. September 14, 2026 falls in ISO week 38.
Frequently Asked Questions
How are months and years added to a date?
Adding months or years shifts the calendar month/year while keeping the day of month where possible; if the resulting month is shorter (e.g. adding a month to Jan 31), the date is adjusted to the last valid day of that month.
Does "days between" include both endpoints?
No — it returns the number of days separating the two calendar dates (e.g. Jan 1 to Jan 2 is 1 day), not an inclusive day count. For an inclusive count, use our Days Between Dates Calculator.
What week-numbering system does this use?
ISO 8601, the international standard: weeks start on Monday and end on Sunday, and the first week of the year is whichever week contains that year's first Thursday. This means the first few days of January can sometimes fall in the last week of the previous year, and vice versa in late December.
Can I calculate a date in the past?
Yes. Choose "Subtract" in Add/Subtract mode, or simply enter your two dates in any order in Count Days mode — the calculator handles both directions.
How do I count only workdays, or add business days to a date?
Use the Business Days Calculator, which skips Saturdays and Sundays when counting or adding days.