calendar calculator.
A professional, standards-based calendar calculator for planners, lawyers, accountants, and operations teams. Compute precise date differences, add or subtract months/years safely across end-of-month boundaries, count business days with optional holidays, and retrieve weekday & ISO week numbers.
Calendar Calculator Interface
Results
Authoritative Data Source & Methodology
Primary standards referenced:
- ISO 8601:2019 — Date and time — Representations for information interchange (weeks start Monday; week 1 contains the first Thursday).
- Gregorian calendar rules — Leap year if divisible by 4, except end-of-century years not divisible by 400.
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
See also: ISO overview and ISO week date summary.
The Formula Explained
Date difference (days):
\[ \Delta d = \mathrm{days}(d_2) - \mathrm{days}(d_1) \]
Y–M–D breakdown (proleptic Gregorian):
\[ \Delta y = y_2 - y_1,\quad \Delta m = m_2 - m_1,\quad \Delta d' = d_2 - d_1 \] Adjust \(\Delta m,\Delta d'\) by borrowing/carrying based on month lengths and leap-year rules.
Add/Subtract with end-of-month safety:
\[ \mathrm{add}(y,m,d; \Delta y,\Delta m,\Delta d) = \mathrm{clampEOM}\bigl(y+\Delta y,\; m+\Delta m,\; d\bigr) + \Delta d \] where \(\mathrm{clampEOM}\) sets the day to the month's last valid day if needed.
Business days, excluding weekends & holidays:
\[ \mathrm{bizdays}(d_1,d_2,H)=\sum_{t=d_1}^{d_2-1}\mathbf{1}\bigl(\mathrm{weekday}(t)\in\{1,\dots,5\}\land t\notin H\bigr) \]
ISO week number (ISO 8601):
\[ \mathrm{week}(d)=\mathrm{ISOWeekNumber}(d),\quad \mathrm{weekday}_{\mathrm{ISO}}\in\{1\ldots7\} \]
Glossary of Variables
- Start/End date: The two dates used for difference or business-day counting.
- Base date: The date to which years/months/days are added or subtracted.
- Years / Months / Days: Integer components applied to the base date; negative values are supported via the subtract option.
- Business days: Monday–Friday, excluding listed holidays.
- ISO week: Week numbering per ISO 8601; weeks start Monday; week 1 holds the first Thursday.
How It Works: A Step-by-Step Example
Add one month to January 31, 2024, then 10 days
- Base date: 2024-01-31. Add 0 years, 1 month, 10 days.
- Month add yields February 31 → clamped to February 29 (leap year).
- Add 10 days → 2024-03-10.
The clamping uses the \(\mathrm{clampEOM}\) rule above to guarantee valid results in shorter months.
Frequently Asked Questions
Which calendar system does this tool use?
Proleptic Gregorian with ISO 8601 conventions for weeks/dates.
How are business days counted?
It excludes weekends and any holidays you provide. The interval is start-inclusive, end-exclusive by default.
What happens at the end of months?
When adding months/years, if the original day isn’t present in the target month, the date clamps to the month’s last valid day.
Does it support negative differences?
Yes, the tool reports signed day differences and also shows absolute values.
Are time zones considered?
No, this is a pure date calculator—no time-of-day or DST effects.
Is ISO week the same as US week numbers?
Not necessarily; ISO weeks start Monday and can differ from locale-specific systems.
Tool developed by Ugo Candido. Content verified by CalcDomain Editorial Board.
Last accuracy review: