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Date & Time Calculator

In-browser only ISO 8601 Proleptic Gregorian
C copy · R reset

Type MM/DD/YYYY or pick a day below.

Operation

Whole number, 0-9999.

Unit

Jun 19, 2026 + 1 Days

Saturday
June 20, 2026
MM/DD/YYYY06/20/2026

All calculations happen in your browser - no dates are sent anywhere.
Related time tools View all time tools

Overview

What this tool does

Three modes in one: add or subtract an interval from a start date, compute the calendar duration between two date-times, or measure the elapsed time between two clock readings — all calculated instantly in your browser.

  • Add or subtract days, weeks, months, or years from any date
  • Live calendar picker with month navigation
  • Date duration mode between two date-times
  • Time duration mode between two clock readings
  • ISO 8601 duration output for all modes
  • All calculations run in your browser
  • No dates or times leave the page
  • Works offline after first load
  • Nothing stored on any server.
  • Nothing is stored or uploaded
  • Project deadlines and contract end dates
  • Pregnancy due-date estimates
  • Travel itinerary planning
  • Payroll and billing date calculations
  • Scheduling intervals for recurring events

How to use the Date & Time Calculator

  1. 1
    Choose a mode

    Select Add / Subtract to offset a date, Date duration to measure between two date-times, or Time duration to measure between two clock times.

  2. 2
    Enter your start date

    Type a date in MM/DD/YYYY format or click any day in the calendar picker. The result updates instantly.

  3. 3
    Set the interval

    Choose Add or Subtract, enter a quantity, and pick a unit (Days, Weeks, Months, or Years). The result appears immediately — no button needed.

  4. 4
    Copy the result

    Click Copy result or press C to copy the calculated date to your clipboard. Press R to reset to today.

Frequently asked questions

How does month-end anchoring work?
When you add or subtract months and the resulting day does not exist in the target month (e.g. January 31 + 1 month), the result is clamped to the last valid day of that month (February 28 or 29). This follows standard calendar arithmetic.
Does the tool handle leap years correctly?
Yes. The tool uses the Proleptic Gregorian calendar, which correctly identifies century years divisible by 400 as leap years (2000 is a leap year; 1900 and 2100 are not). An inline error appears for invalid dates like February 29, 1900.
What date formats are supported?
The date fields accept MM/DD/YYYY. The result also shows the ISO 8601 date. The Date duration and Time duration modes output ISO 8601 duration strings (P2M29DT8H30M, PT8H30M, etc.).
What does the Overnight option do in Time duration mode?
If the To time is earlier than the From time, Roll to next day adds 24 hours to treat it as crossing midnight (e.g. 10:00 PM to 2:00 AM = 4 hours). Same day shows a hint instead of computing a negative duration.
Is my data private?
Yes. All calculations happen entirely in your browser. No dates, times, or results are sent to a server. The tool works offline after the page has loaded.
What is the supported date range?
The tool supports dates from year 1 to year 9999. An error message appears if a calculation would produce a date outside this range.