Date Offset Calculator
Add or subtract days, weeks, months, and years from any date with detailed result information
Input
Output
Readme
What is a date offset?
A date offset is a fixed or variable amount of time added to or subtracted from a reference date to produce a new date. Offsets can be expressed in many units — years, months, weeks, days, hours, minutes, seconds, or even milliseconds — and they are fundamental to scheduling, project planning, and time-based calculations. For example, adding 90 days to a contract signing date gives you the delivery deadline, while subtracting 2 weeks from a launch date tells you when final testing must begin.
Date arithmetic is trickier than it looks. Months have different lengths, leap years add an extra day, and business-day calculations must skip weekends. Manually juggling these edge cases is error-prone, especially when multiple units are combined — such as "add 1 year, 3 months, and 15 business days." A dedicated calculator handles all the calendar quirks so you get an accurate result every time.
Tool description
The Date Offset Calculator lets you pick any starting date and time, choose whether to add or subtract, and enter offsets across twelve different time units simultaneously. It instantly computes the resulting date along with useful metadata like the day of the week, ISO week number, day-of-year index, whether the result falls in a leap year, and the Unix timestamp in milliseconds.
Examples
| Start date | Operation | Offset | Result |
|---|---|---|---|
| 2026-01-01 | Add | 6 months, 15 days | Thursday, July 16, 2026 |
Features
- Combined multi-unit offsets — mix years, quarters, months, weeks, days, business days, hours, minutes, seconds, and milliseconds in a single calculation
- Business-day aware — automatically skips Saturdays and Sundays when computing business-day offsets
- Rich result details — displays day of week, day of year, ISO week number, leap-year status, and Unix timestamp alongside the result date
- Bidirectional operation — toggle between adding and subtracting offsets with one click
- Millisecond precision — supports datetime input and output down to the millisecond for high-accuracy scenarios
Use cases
- Project management — calculate deadlines by adding business days or calendar weeks to a project start date, accounting for weekends automatically
- Legal and financial planning — determine contract expiration dates, payment due dates, or notice periods by offsetting from a reference date with precise units
- Event scheduling — find exact dates for recurring milestones such as "6 months and 10 days after launch" without manual calendar counting
Options explained
| Option | Description |
|---|---|
| Start date | The reference date and time from which the offset is applied |
| Operation | Choose Add (+) to move forward in time or Subtract (−) to move backward |
| Years / Quarters / Months | Calendar-based offsets; quarters are converted to 3-month increments |
| Weeks / Days | Straightforward calendar-day offsets (weeks = 7 days each) |
| Business days | Weekday-only offset that skips Saturdays and Sundays |
| Hours / Minutes / Seconds / Milliseconds | Sub-day time offsets for precise time calculations |
How it works
The calculator applies offsets in a specific order to ensure predictable results:
- Years are added or subtracted first by adjusting the year component of the date.
- Quarters and months are combined and applied next, shifting the month component.
- Weeks and days are combined into total days and applied to the day component.
- Business days are then iterated one day at a time, skipping any Saturday or Sunday.
- Hours, minutes, seconds, and milliseconds are applied last for sub-day precision.
This order matters because calendar-based units (years, months) can shift the date in ways that affect subsequent day-level calculations. Applying them first avoids unexpected interactions.
FAQ
Why does adding 1 month to January 31 give a different result than expected?
Months have varying lengths. JavaScript's Date object handles overflow by rolling forward — so January 31 + 1 month becomes March 3 in non-leap years (since February only has 28 days). This is standard date arithmetic behavior.
Does the business-day calculation account for public holidays? No. Business days skip only Saturdays and Sundays. Public holidays vary by country and region, so they are not included in the calculation.
Can I enter negative numbers in the offset fields? The offset fields accept only non-negative values. Use the Operation dropdown to switch between adding and subtracting instead.