About

Date arithmetic errors cascade. A lease ending one day late triggers penalty clauses. A medication schedule shifted by 1 day compounds across 30 doses. This calculator performs exact day addition and subtraction on any calendar date, correctly handling leap years (where February has 29 days when the year is divisible by 4, except centuries not divisible by 400), month-boundary rollovers, and Daylight Saving Time transitions. It does not approximate. It computes using the proleptic Gregorian calendar as implemented by the ECMAScript Date specification (ECMA-262 §21.4).

The business days mode excludes Saturdays and Sundays from the count, which matters for contractual deadlines governed by statutes like the UCC or FIDIC. Note: this tool does not account for public holidays, which vary by jurisdiction. Pro Tip: for legal deadlines, always confirm your local holiday calendar separately and add a 1-day buffer for filing systems that close early.

Formulas

The result date D_result is computed by direct calendar day offset from a start date D_start:

D_result = D_start + n

where n is the signed number of calendar days (n > 0 for future, n < 0 for past). The engine internally calls Date.setDate(Date.getDate() + n), which handles month and year rollovers natively.

For business days mode, the algorithm iterates day-by-day, incrementing a counter k only when the current day-of-week w satisfies:

w ∉ {0, 6}

where 0 = Sunday and 6 = Saturday (ECMAScript convention). The loop terminates when k = |n|.

The leap year predicate L(y) used internally:

{

TRUE if y mod 400 = 0FALSE if y mod 100 = 0TRUE if y mod 4 = 0FALSE otherwise

The duration breakdown converts the absolute difference Δt in milliseconds to weeks and remaining days:

W = floor(Δt604800000), R = Δt mod 60480000086400000

where 604800000 = 7 × 86400000 ms (one week) and 86400000 ms = 24 × 60 × 60 × 1000 (one day).

Reference Data

Month	Days	Cumulative (Non-Leap)	Cumulative (Leap)	Notes
January	31	31	31	Start of Q1
February	28 / 29	59	60	Leap day: Feb 29
March	31	90	91	Start of Q2 (some fiscal)
April	30	120	121	Start of Q2
May	31	151	152	-
June	30	181	182	Mid-year
July	31	212	213	Start of Q3
August	31	243	244	-
September	30	273	274	Start of Q4 (some fiscal)
October	31	304	305	Start of Q4
November	30	334	335	-
December	31	365	366	End of year
Leap Year Reference
Year 2024	366 days (leap)			Divisible by 4
Year 2025	365 days			Not divisible by 4
Year 2100	365 days			Century, not ÷ 400
Year 2000	366 days (leap)			Century, ÷ 400
Common Periods
Fortnight	14 days			2 weeks
Quarter	90 - 92 days			Varies by months
Semester	182 - 183 days			Half-year
Fiscal Year (US)	365 / 366 days			Oct 1 - Sep 30

Frequently Asked Questions

The calculator uses the native ECMAScript Date engine which correctly implements the Gregorian leap year rule: a year is a leap year if divisible by 4, except for century years which must also be divisible by 400. So adding 365 days from January 1, 2024 (a leap year) yields December 31, 2024 - not January 1, 2025 - because 2024 has 366 days. The rollover is handled automatically.

No. Business days mode excludes only Saturdays and Sundays (ISO weekday indices 6 and 0). Public holidays vary by country, state, and even municipality. For legal or contractual deadlines, cross-reference the result with your jurisdiction's official holiday calendar and add the corresponding number of additional days manually.

DST shifts cause certain calendar days to have 23 or 25 hours. The calculator adds calendar days, not fixed 24-hour periods. Adding 1 day to March 9, 2025 at midnight in US Eastern time yields March 10, 2025 at midnight, even though only 23 hours have elapsed. This is the correct behavior for date arithmetic as opposed to duration arithmetic.

Yes. Enter a negative number in the days field or use the Subtract toggle. The same algorithm runs in reverse: the Date engine correctly rolls back across month and year boundaries. For example, subtracting 60 days from March 1, 2025 yields December 31, 2024.

The calculator accepts up to ±999,999 days (approximately ±2,739 years). The ECMAScript Date object supports dates from April 20, 271821 BCE to September 13, 275760 CE (±8.64 × 10¹⁵ milliseconds from epoch). Values producing dates outside this range will display an error.

The breakdown decomposes the absolute day count into complete weeks (groups of 7) plus remaining days. If you add fewer than 7 days, the weeks component is 0. For example, adding 10 days shows 1 week and 3 days.