Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Resulting date
Monday, September 14, 2026
ISO format
2026-09-14
Day of the week
Monday
Total days shifted
30
Weeks shifted
4.29
Day of the year
257
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter start date.
- 2Enter days to add.
- 3Enter weeks to add.
- 4Enter months to add.
- 5Enter years to add.
- 6Read your resulting date on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Useful for deadlines, contract notice periods, and return windows. Month arithmetic clamps to the last valid day, so adding one month to January 31 gives February 28 or 29.
Worked example
Using the values the calculator loads with:
Inputs
- Start date: 2026-08-15
- Days to add: 30
- Weeks to add: 0
- Months to add: 0
- Years to add: 0
Results
- Resulting date: Monday, September 14, 2026
- ISO format: 2026-09-14
- Day of the week: Monday
- Total days shifted: 30
- Weeks shifted: 4.29
- Day of the year: 257
What each field means
Inputs
- Start date
- The reference date for this calculation. Defaults to today unless you change it.
- Days to add
- The days to add used in the calculation. Starts at 30 so you have a working example on load.
- Weeks to add
- The weeks to add used in the calculation. Starts at 0 so you have a working example on load.
- Months to add
- The months to add used in the calculation. Starts at 0 so you have a working example on load.
- Years to add
- The years to add used in the calculation. Starts at 0 so you have a working example on load.
Results
- Resulting date
- Returned as a plain value and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- ISO format
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Day of the week
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total days shifted
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Weeks shifted
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Day of the year
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the add or subtract days from a date work?
Useful for deadlines, contract notice periods, and return windows. Month arithmetic clamps to the last valid day, so adding one month to January 31 gives February 28 or 29. The underlying maths is: Result = Start date + days + weeks × 7 + months + years.
What do I need to enter?
5 values: start date, days to add, weeks to add, months to add, and years to add. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the resulting date result mean?
It is returned as a plain value and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Date Add / Subtract Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/add-days-to-date
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/add-days-to-date" target="_blank" rel="noopener">Date Add / Subtract Calculator — RevenueLab</a> (2026).</p>
Source: [Date Add / Subtract Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/add-days-to-date) (2026).