Which calendar system does this tool use?

It uses the proleptic Gregorian calendar, aligned with ISO 8601 conventions for weeks and dates.

How are business days counted?

Weekends (Saturday and Sunday) are excluded by default. You can also exclude custom holidays you enter.

Can I add months and years accurately across end-of-month boundaries?

Yes. Adding months/years clamps to the last valid day of the target month when the original day does not exist (e.g., adding 1 month to Jan 31 becomes Feb 29 in leap years or Feb 28 otherwise).

What about leap years?

Leap years follow the Gregorian rules: divisible by 4, except centuries not divisible by 400.

How is ISO week number defined?

Per ISO 8601, weeks start on Monday, and week 1 is the week containing the year’s first Thursday.

Can I paste holidays from a spreadsheet?

Yes. Paste dates as YYYY-MM-DD, one per line; duplicates and blanks are ignored.

Does the tool consider time zones?

Calculations are date-based (no time-of-day) and ignore time zones for consistency.

calendar calculator — Add/Subtract Dates, Day Counter, Business Days

Calendar calculator to add or subtract days, months, or years; find the difference between dates; count business days (with holidays); and get weekday & ISO week.

Mode

Switch between calculations without leaving the card. Inputs update automatically.

Date difference

Start date

End date

Range counts from the start date inclusive and end date exclusive.

Add or subtract a duration

Base date

Operation Subtract duration

Leave unchecked to add the values below.

Years

Months

Days

Month/year adjustments clamp to month ends when necessary, then days are added.

Business days counter

Start date

End date

Holidays (YYYY-MM-DD, one per line)

Week number & weekday

Select a date

ISO weeks start on Monday and week 1 contains the year’s first Thursday.

Result summary

Ready when you are.

Choose a mode and press Calculate to see the breakdown here.

How to Use This Calculator

Pick the mode that matches your question, fill out the dates or duration, and hit Calculate. All fields are built-in inputs, and every change is validated before the result card updates.

Modes in brief

Date Difference: Measures days between two dates (end-exclusive) plus a Y/M/D breakdown.
Add/Subtract: Moves a base date forward or backward by Y/M/D values using end-of-month clamping.
Business Days: Counts weekdays between dates while allowing holiday overrides.
Week & Weekday: Reports ISO week number and weekday name for a single date.

Interface tips

The Calculate button always triggers an immediate update. Inputs and selects also update the result with a 100ms debounce so you can fine-tune values without flooding the calculator.

Use the Reset button to restore the default example data and recalculate instantly.

Methodology

The calculator uses the proleptic Gregorian calendar and ISO 8601 logic when interpreting dates. Adding months or years clamps to each month’s last valid day before applying day-level adjustments. Business days exclude weekends by default and honor any holidays you type as YYYY-MM-DD on separate lines.

Days between dates are counted from the start (inclusive) to the end (exclusive); add 1 if you want the inclusive count.
Durations apply month/year arithmetic first, then add or subtract days while respecting month lengths.
ISO weeks always start on Monday; weeks that span calendar years are labeled according to the ISO standard.

The results are estimates produced by deterministic date math and should be used for planning. They are not legal or financial advice—verify important ranges with official calendars if needed.

About the author

Ugo Candido builds practical calculators and guides so readers can explore decisions with transparent, trustworthy models.

Contact: info@calcdomain.com

Editorial policy

CalcDomain content is created for education and reviewed for clarity and accuracy. We do not accept paid placements that change calculator outputs. Inputs and assumptions remain visible so you can verify the logic yourself.

Formulas

Key identities used in this engine:

\[\Delta d = \mathrm{days}(d_2) - \mathrm{days}(d_1)\]
\[\Delta y = y_2 - y_1,\quad \Delta m = m_2 - m_1,\quad \Delta d' = d_2 - d_1\]
\[\mathrm{add}(y,m,d; \Delta y,\Delta m,\Delta d) = \mathrm{clampEOM}(y+\Delta y,\; m+\Delta m,\; d) + \Delta d\]
\[\mathrm{bizdays}(d_1,d_2,H)=\sum_{t=d_1}^{d_2-1} \mathbf{1}(\mathrm{weekday}(t)\in\{1,\dots,5\}\land t\notin H)\]
\[\mathrm{week}(d)=\mathrm{ISOWeekNumber}(d),\quad \mathrm{weekday}_{\mathrm{ISO}}\in\{1\ldots7\}\]

Variables follow standard calendar definitions (years, months, days, holidays, ISO weekdays).

Citations

ISO overview — iso.org · Accessed 2026-01-19
https://www.iso.org/iso-8601-date-and-time-format.html
Home — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/
Everyday Life — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/lifestyle-everyday
Time and Date — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/subcategories/time-date
ISO week date summary — en.wikipedia.org · Accessed 2026-01-19
https://en.wikipedia.org/wiki/ISO_week_date

Changelog

0.1.0-draft — 2026-01-19: Initial draft release (audit spec style).

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

Version 1.5.0