Date Calculator: Find Duration & Add/Subtract Dates

How to Use This Calculator

Select the mode that matches your goal, enter the relevant dates, and click Calculate. The duration tab compares two dates while the add/subtract tab instantly moves the start date by the amounts you enter.

• Choose either Calculate Duration or Add / Subtract Time so the inputs below match your task.
• When finding a duration, supply both a start and end date and decide whether the end day should be counted.
• When altering a date, pick add or subtract, then enter years, months, weeks, or days.
• Hit Calculate or make edits—results rerun automatically after a short delay.
• Reset restores today's date plus clears all time offsets.

Methodology

The engine parses ISO date inputs so time zone quirks are avoided. For duration mode it swaps dates when needed, converts milliseconds to whole days, and walks the calendar to count years, months, and days. Including the end day simply adds one more day before adjusting the breakdown.

For add/subtract mode the calculator adds years and months first, guarding against month-end rollovers by snapping to the last valid day when the original day does not exist. Weeks convert to days, and the final day-level adjustment applies with the chosen direction.

Numbers are rendered with localized thousands separators and the interface never shows NaN because every input is validated before rendering results.

Full original guide (expanded)

$TotalDays = \lfloor \frac{Timestamp_{End} - Timestamp_{Start}}{1000 \times 60 \times 60 \times 24} \rfloor