Date & Time
Age, duration & timezone calculators
Date & Time calculators
7 tools for Age, Date difference, Time zones and more — accurate, instant, and free.
Reckonist’s Date & Time calculators turn calendar questions into exact answers — your precise age, the duration between two dates, adding or subtracting days and working days, and converting times across zones. Each one runs in your browser — accurate, free, no sign-up, and your dates never leave your device.
Date & time math4
Date Difference Calculator
Days, weeks and a calendar year/month/day breakdown between two dates, plus a business/working-days mode (weekdays minus US federal holidays with observed-date rules) and an inclusive/exclusive endpoint toggle. DST-safe UTC math, computed entirely in your browser.
Date Add/Subtract Calculator
Add or subtract days, weeks, months, years or business days from a date and get the resulting date and weekday. Month/year math clamps to valid dates (Jan 31 + 1 month = Feb 28, never Mar 3); business days skip weekends and US federal holidays. DST-safe UTC math, computed entirely in your browser.
Time Duration Calculator
Hours and minutes between two times, shown as H:MM, total minutes and decimal hours (for timesheets). Overnight-safe — a 22:00 → 06:00 shift reads 8 hours, not −16 — with an unpaid-break deduction. Pure clock math, computed entirely in your browser.
Week Number Calculator
The ISO-8601 week number for any date — Monday-start, week 1 is the week containing the year’s first Thursday (Jan 4) — plus the ISO year, the week’s Monday–Sunday date range and weeks-in-year (52 or 53). Matches Excel/Sheets ISOWEEKNUM, and handles the year-boundary edge where the ISO year differs from the calendar year. Computed entirely in your browser.
Age & countdowns2
Age Calculator
Exact age as years, months and days from a birth date to any date, with total days and weeks lived and a next-birthday countdown. A baby-age mode reads the age in weeks and months, and a corrected-age mode adjusts a preterm infant for prematurity (chronological − (40 − gestational weeks), to ~24 months). Correct month-length borrow, leap-year and Feb-29 handling on DST-safe UTC math, computed entirely in your browser.
Countdown Calculator
A live countdown — days, hours, minutes and seconds — to a holiday, season, year or custom date, with a days-since count-up mode. Moving holidays are computed, not hardcoded: Western Easter via the Meeus/Jones/Butcher computus and US Thanksgiving as the 4th Thursday of November, so the date is right every year and auto-rolls to the next occurrence once passed. Shows the weekday the event falls on, a weeks-and-days breakdown, and an add-to-calendar (.ics) download. DST-safe UTC math, computed entirely in your browser.