About

Misreading date intervals causes project overruns, contract disputes, and scheduling failures. A span of 90 calendar days contains only 64 business days assuming standard weekends. This tool computes precise durations using Gregorian calendar rules, accounting for variable month lengths (28 - 31 days) and leap year corrections. The overlap detection algorithm identifies conflicting date ranges in O(n²) comparisons.

Duration breakdown converts total milliseconds into years, months, and days via successive subtraction rather than simple division. This produces human-readable output matching how people count intervals. Business day calculations exclude weekends but not regional holidays. For compliance with ISO 8601 or project management standards like PMBOK, verify results against your organization's working calendar.

Formulas

The precise duration between two dates d₁ and d₂ requires calendar-aware computation. Total milliseconds provide the base measurement.

Δt = d 2 - d 1

Conversion to larger units uses integer division with remainders.

days = floor (Δt 86,400,000)

Business days exclude weekends via day-of-week filtering.

B = d 2 \sum d = d 1 [dow (d) \notin {0, 6}]

Where dow(d) returns 0 for Sunday through 6 for Saturday. Overlap detection between intervals A and B uses.

overlap = \neg (A end < B start \lor B end < A start)

The year-month-day breakdown subtracts components sequentially, borrowing from larger units when days become negative, similar to manual calendar counting.

Reference Data

Interval Type	Days	Business Days	Hours	Common Use
1 Week	7	5	168	Sprint cycles, weekly reports
2 Weeks	14	10	336	Payroll periods, biweekly sprints
1 Month (avg)	30.44	22	730	Subscription billing, monthly reports
1 Quarter	91.31	65	2,191	Fiscal quarters, OKR cycles
6 Months	182.63	130	4,383	Performance reviews, lease terms
1 Year (common)	365	261	8,760	Annual contracts, fiscal years
1 Year (leap)	366	262	8,784	Leap year calculations
5 Years	1,826	1,305	43,830	Strategic planning, warranties
10 Years	3,652	2,609	87,660	Long-term investments
January	31	23	744	Fixed 31-day month
February (common)	28	20	672	Shortest month
February (leap)	29	21	696	Leap year February
30-day month	30	22	720	Apr, Jun, Sep, Nov
31-day month	31	23	744	Jan, Mar, May, Jul, Aug, Oct, Dec
Work week	5	5	40	Standard 8-hour days
Notice period (US)	14	10	336	Two-week notice standard
Trial period	90	64	2,160	Probationary employment
Semester	120	86	2,880	Academic semesters
Statute limit (3yr)	1,095	782	26,280	Legal statute of limitations

Frequently Asked Questions

The algorithm subtracts years first, then months, then days. When the day component becomes negative, it borrows from the previous month by adding that month's actual length. For example, from January 31 to March 2 yields 1 month and 2 days, not 30 days, because February is accounted for based on whether it is a leap year.

No. Business days exclude only Saturdays and Sundays. Public holidays vary by jurisdiction and year. For precise business day counts in financial or legal contexts, subtract known holidays from the result manually. A typical year has 10-15 federal holidays depending on the country.

Two intervals overlap if neither ends before the other begins. The overlap duration equals the difference between the later start date and the earlier end date. Zero or negative results indicate no overlap. This calculation runs in O(n²) time for n intervals.

The visualizer automatically normalizes reversed dates. If the end date precedes the start date, it swaps them for display and calculation. Duration values are always positive. This prevents negative day counts that would confuse interpretation.

JavaScript Date objects use 64-bit floating-point milliseconds since January 1, 1970. This supports dates roughly 285,000 years into the past or future. Practical accuracy degrades beyond several thousand years due to calendar reform assumptions. Historical dates before 1582 CE do not account for the Julian-to-Gregorian transition.

Leap years add one day to February. A year is a leap year if divisible by 4, except century years must also be divisible by 400. Thus 2000 was a leap year, but 1900 was not. The calculator checks this rule when computing month boundaries and annual totals.