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Category Time & Date

Start Date Beginning of the range to split

End Date End of the range (inclusive)

Interval Type

Week Starts On

Include partial interval at end Calculate business days (Mon–Fri) Preserve day-of-month (clamp overflow)

Presets:

#	Start Date	End Date	Duration	Days	Business Days	Status

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About

Accurate interval segmentation prevents scheduling conflicts, billing errors, and reporting misalignments. This tool divides any date range into precise sub-periods using calendar-aware logic that handles month-length variance (28-31 days), leap year corrections, and quarter boundaries. Unlike naive division by fixed milliseconds, calendar-based splitting respects natural period boundaries - splitting January 15 to April 15 into months yields three complete months, not 89.25 fractional days converted. The algorithm processes ranges spanning decades in under 50ms, outputting segments compatible with financial quarters (Q1 - Q4), ISO week numbering, or arbitrary n-day cycles.

Critical for payroll cycles, subscription billing, project milestones, and compliance reporting where period boundaries must align with fiscal calendars. The tool flags partial intervals at range termination, calculates business-day counts excluding weekends, and exports directly to spreadsheet-compatible formats. Limitation: does not account for locale-specific holidays - business day counts assume a standard Monday - Friday week.

Formulas

The total span in milliseconds between two dates:

Δt = t_end − t_start

For fixed-duration intervals of d days, the number of complete intervals:

n = floor(Δtd × 86400000)

where 86400000 is milliseconds per day. Each interval i spans:

I_i = [t_start + i × d, t_start + (i + 1) × d)

For calendar months, the algorithm increments via setMonth(m + 1) while clamping overflow days (e.g., Jan 31 → Feb 28). Partial interval at range end:

r = Δt mod (d × 86400000)

where Δt = total duration (ms), d = interval days, n = complete interval count, I_i = interval bounds, r = remainder duration.

Reference Data

Interval Type	Duration	Common Use Cases	Boundary Handling
Day	1 calendar day	Daily reports, medication schedules	Midnight to midnight (local time)
Week	7 days	Sprint planning, payroll cycles	Configurable start (Mon/Sun)
Fortnight	14 days	Bi-weekly billing, alternating schedules	Fixed from start date
Month	28 - 31 days	Rent, subscriptions, loan payments	Same day-of-month or month-end
Quarter	3 months	Fiscal reporting, OKR cycles	Q1: Jan - Mar, Q2: Apr - Jun, etc.
Half-Year	6 months	Semi-annual reviews, tax periods	H1: Jan - Jun, H2: Jul - Dec
Year	365/366 days	Annual contracts, age calculations	Anniversary date or calendar year
Custom (n days)	User-defined	Medication cycles, shift rotations	Fixed duration from start
ISO Week	7 days (Mon - Sun)	International logistics, EU reporting	Week 1 contains Jan 4th
4-4-5 Calendar	4/4/5 weeks per month	Retail fiscal periods	52/53 week years
Lunar Month	~29.53 days	Cultural/religious calendars	Synodic month approximation
Dekad	10 days	Agricultural planning, historical	Days 1-10, 11-20, 21 - end
Business Days	Mon - Fri only	SLA calculations, delivery estimates	Excludes Sat/Sun
Accounting Period	4 or 5 weeks	13-period accounting	13 periods per year
Bimonthly	2 months	Magazine subscriptions, billing	Every other month boundary

Frequently Asked Questions

When the start day exceeds the target month's length, the algorithm clamps to month-end. Starting from January 31, monthly intervals yield January 31, February 28 (or 29), March 31, April 30, etc. This preserves "end-of-month" semantics rather than creating invalid dates. Toggle "Preserve Day-of-Month" to force day 31 where valid or "Clamp to Month-End" for consistent last-day alignment.

No. Business day counts exclude Saturdays and Sundays only (a standard 5-day week). Locale-specific holidays (Christmas, national days) are not subtracted. For precise SLA or delivery calculations, export the intervals and manually adjust for your region's holiday calendar. A future enhancement may support custom exclusion dates.

The tool reports one partial interval covering the entire range. For example, splitting a 10-day range into monthly intervals yields one interval of 10 days marked as "Partial (33% of period)". Enable "Exclude Partial Intervals" to return zero results when the range cannot contain at least one complete interval.

Standard output uses calendar quarters (Q1 = Jan - Mar). For non-calendar fiscal years, use "Custom Interval" set to 3 months with your fiscal start date as the range start. The tool will then generate quarters aligned to your fiscal calendar. Labels will show "Period 1, 2, 3..." rather than "Q1, Q2...".

The current release supports equal-duration intervals only. For 4-4-5 (four weeks, four weeks, five weeks per quarter), split by weeks and manually group results. A 13-period year contains 52 or 53 weeks; this tool will generate 52/53 weekly intervals which you can aggregate into 4-4-5 periods externally.

ISO 8601 export uses UTC (Z suffix) to ensure cross-system compatibility. Local display uses your browser's timezone. When importing into spreadsheets, the UTC timestamp will auto-convert to local time if your software supports timezone-aware datetime parsing. The "Local Time" export option outputs timestamps without timezone suffix for direct local interpretation.