Measure or shift a date interval
Find the elapsed span or move a date by a precise offset.
Date Duration Calculator
Add or subtract days, weeks, months, and years from any calendar date.
Live
Parameters
R Reset
The start date of the period.
Number of months to add or subtract.
601,200
Number of weeks to add or subtract.
005,200
Number of days to add or subtract.
0036,500
The investment time horizon in years.
001,000
Analytics Dashboard
Target Calendar Date
Friday, December 18, 2026
Calculated ISO Date2026-12-18
Operation PerformedAdded Time
Mathematical Explanation
Adds or subtracts years, months, weeks, and days recursively from the start date to identify the final target date.
How to Use the Date Duration Calculator
Add or subtract time intervals from any starting date with our free date duration calculator. Whether you are tracking pregnancy milestones, scheduling project milestones, or finding historical dates, this tool makes calculations simple. Input a starting date, select whether to add or subtract time, and define the days, weeks, months, or years to find the target date.
Input variables:
- Start Date: Start Date
- Action: Action
- Years: Years
- Months: Months
- Weeks: Weeks
- Days: Days
Date Duration Calculator Explained: Formula, Math, and Applications
Learn the mathematics, formulas, and real-world applications behind the Date Duration Calculator.
Read Full Guide →How do you add months to a date when month lengths vary?
Our calculator handles varying month lengths by calculating standard calendar months. If the target month has fewer days than the starting day (e.g. adding 1 month to August 31st), the calculator adjusts the target date to the last day of the target month (September 30th).
How can I use this tool to calculate pregnancy due dates?
A standard pregnancy due date is calculated by adding 280 days (or 40 weeks) to the first day of the last menstrual period (LMP). You can select your LMP as the start date and add 40 weeks using this tool.
Does this tool calculate across historical eras?
Yes. The calculator supports standard calendar mathematics across past and future eras, making it a reliable tool for historical timelines and research.
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