Does the calculator account for leap years?

Yes. All calculations use the proleptic Gregorian calendar, which includes leap-year rules (years divisible by 4, except centuries not divisible by 400).

What does inclusive counting mean?

If enabled, the start date is counted as day one. For example, adding 10 days inclusive is equivalent to adding 9 days exclusive.

How are business days calculated?

Business days are Monday through Friday. If enabled, weekends are skipped. You can also exclude custom holidays you paste into the tool.

Which calendar system is used?

The proleptic Gregorian calendar. Dates are handled as date-only values using UTC to avoid daylight saving time issues.

How should I enter holidays?

Enter one ISO date per line (YYYY-MM-DD). Invalid dates are ignored and flagged inline.

What is the maximum number of days?

Up to 100,000 for calendar-day math. For business days, up to 10,000 to ensure instant performance.

Add/Subtract Days from a Date Calculator

Professional date calculator to add or subtract days from a date. Supports business days, weekends/holiday exclusion, inclusive counting, and WCAG 2.1 AA accessible UI.

Date math

Add or subtract calendar or business days, in any combination.

Add Subtract

Start date

Number of days

Inclusive counting (start date counts as day 1) Business days (skip weekends)

Holidays to exclude

Separate dates with newlines or commas. Invalid lines are ignored.

Resulting date

—

Days entered —

Mode —

Excluded days —

Detail —

Results update instantly when you click Calculate. Calendar math uses strict UTC dates and does not rely on local time.

How to Use This Calculator

Pick whether you want to add or subtract days, enter a start date, set the number of days, then click Calculate. Optional controls let you count inclusively, treat only weekdays as business days, and paste holidays you want to skip.

Inclusive counting treats the start date as day one, so adding 10 days inclusive is equivalent to adding nine exclusive.
Business-day mode skips Saturdays, Sundays, and any ISO dates you paste into the Holidays box.
The results card reports the ISO date, day of the week, how many exclusions were applied, and a quick detail of the math used.

Methodology

All calculations run on UTC dates so Daylight Saving Time and time zones cannot shift the answer. Calendar math simply adds or subtracts the requested days, adjusting for inclusive counting when necessary. Business days advance one day at a time, skipping weekends and the supplied holidays until the target number of workdays is reached.

Validation enforces ISO dates, whole-day counts, and reasonable limits (100,000 calendar days, 10,000 business days) to keep performance instant.

Full original guide (expanded)

Step-by-step example drawn from the original walkthrough:

Choose the operation you need (Add or Subtract).
Enter the start date, like 2025-01-15.
Set the number of days, switch on business-day logic, and paste holidays such as 2025-02-17.
Click Calculate to get the resulting date and a breakdown of exclusions.

FAQ from the legacy copy

Does the calculator account for leap years?: Yes. All calculations follow the proleptic Gregorian calendar, including standard leap-year behavior.
What does inclusive counting mean?: Inclusive counting counts the start date as day one, so adding 10 days inclusive equals adding 9 exclusive.
How are business days calculated?: Business days are Monday through Friday. Weekends are skipped, and any holidays you list are also excluded.
Which calendar system is used?: The proleptic Gregorian calendar is used via UTC date-only math to avoid daylight saving side effects.
How should I enter holidays?: Paste one ISO date per line (YYYY-MM-DD). Invalid dates are ignored but noted below the field.
What is the maximum number of days?: Use up to 100,000 for calendar math and up to 10,000 for business days to keep the interface responsive.

Formulas

Calendar math: R = S + o · d, where S is the start date, o is the direction (+1 add, −1 subtract), and d is the whole number of days.

Inclusive adjustment: if inclusive counting is enabled and d > 0, compute d' = d − 1 before adding or subtracting.

Business days: R = B⁽ᵈ⁾(S) advances day-by-day, skipping weekends and holidays until d business days are consumed.

Leap years: follow Gregorian rules: a year is leap when divisible by 4, except centuries not divisible by 400.

Variables: start date (ISO), operation (+/−), day count (non-negative integer), holidays (ISO list), inclusive flag, business-day flag.

Citations

iso.org/standard/70907 — ISO 8601 calendar standards, accessed 2026-01-19. https://www.iso.org/standard/70907.html
Everyday Life — CalcDomain lifestyle guides, accessed 2026-01-19. https://calcdomain.com/lifestyle-everyday
Calendrical Calculations — Cambridge University Press, accessed 2026-01-19. https://www.cambridge.org/core/books/calendrical-calculations/6D3EB6B1A8B6C2FA2C8C2583F68C7E1A

Changelog

Version: 0.1.0-draft · Last update: 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

✓ Verified by Ugo Candido Last Updated: 2026-01-19 Version 0.1.0-draft

Version 1.5.0