Dates

Date and time calculators

Every date tool on OpenTools. Date differences, age, durations, working days and deadlines, worked out in your browser as you type.

All 17 of them are listed below. Each one runs inside this tab, so nothing you open or type is uploaded — see the proof.

• Add days to dateAdd calendar days with UTC-stable date arithmetic.
• Age calculatorCalculate calendar age and total elapsed days.
• Anniversary calculatorCount complete years and remaining days; Feb 29 uses Feb 28 when needed.
• Birthday countdownCount calendar days to the next month/day; Feb 29 uses Feb 28 in non-leap years.
• Business-days calculatorCount Monday–Friday dates, excluding the start and including the end.
• Date difference calculatorCount exact calendar days between two dates.
• Day-of-year calculatorEnter a date from year 0100 to 9999 and get its ordinal position in that year, where 1 January is day 1 and 31 December is 365, or 366 in a leap year.
• Duration calculatorCalculate elapsed time between two timestamps with explicit offsets.
• Hours calculatorCalculate hours between two 24-hour times, allowing overnight spans.
• ISO date formatterValidate an ISO timestamp with an explicit offset and normalize it to UTC.
• ISO week-number calculatorTurn a calendar date into its ISO-8601 week, returned as YYYY-Www. The week year can differ from the date’s own year in early January and late December.
• Leap-year checkerEnter a year from 1 to 9999 to see whether it is a leap year under the Gregorian rule: divisible by 4, except century years, unless divisible by 400.
• Meeting-time plannerCompare one proposed instant across multiple IANA time zones.
• Subtract days from dateSubtract calendar days with UTC-stable date arithmetic.
• Timezone converterFormat one absolute timestamp in a selected IANA time zone.
• Workday calculatorAdd or subtract weekdays; public holidays are not included.
• World clockShow one absolute instant in a comma-separated list of IANA time zones.