Week Calculator
### Calendar Chronometry & ISO 8601 Week Standards The **International Organization for Standardization (ISO 8601)** defines the standard calendar week as a 7-day time interval starting on.
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📐 Formula
💡 Practical Example
A 98-day project sprint between August 15 and November 21 comprises exactly 14 full weeks, providing 70 business workdays for project deliverables.
📖 About Week Calculator
Calendar Chronometry & ISO 8601 Week Standards
The International Organization for Standardization (ISO 8601) defines the standard calendar week as a 7-day time interval starting on Monday.
Primary Week Formulas
- Elapsed Calendar Days: D = (Δ t) / (86,400,000 ms)
- Full Weeks and Remaining Days: W = (D) / (7) , D_rem = D ±od 7
- Decimal Weeks: W_decimal = (D) / (7)
- Business Workdays: Count of all intermediate calendar days where Day , Sunday
- Date Shift Formula: Target Date = Start Date ± (N_weeks × 7 days)
How to Use This Calculator
Enter your Calculation Mode — choose from Calculate Weeks Between Two Dates, Add Weeks to a Starting Date, Subtract Weeks from a Date, Start / Reference Date (YYYY-MM-DD) (e.g. 2026-08-15), End Date (for Between Dates Mode) (e.g. 2026-11-21), and Weeks to Add or Subtract (e.g. 14.0 in weeks). Results update instantly as you type — adjust any value to compare scenarios.
The Formula
Formula Reference: Week Calculator
Input variables:
calculationModeSelect — Calculation Mode
startDateInput — Start / Reference Date (YYYY-MM-DD)
endDateInput — End Date (for Between Dates Mode)
numberOfWeeksInput — Weeks to Add or Subtract
Computed outputs:
fullWeeksAndDays — Weeks & Days
decimalWeeks — Total Decimal Weeks
totalCalendarDays — Total Calendar Days
businessWorkdays — Business Workdays (Mon–Fri)
calculatedTargetDate — Target Gregorian Date
Mathematical relationships extracted from calculation logic:
diffMs = d2.getTime - d1.getTime
totalDays = Math.abs(Math.round(diffMs / (1000 60 60 * 24)))
fullW = Math.floor(totalDays / 7)
addDays = Math.round(numWeeks * 7)
sign = mode === 'add_weeks' ? 1 : -1
remD = Math.round((numWeeks - fullW) * 7)
workdays = Math.round(numWeeks * 5)
Standard: NIST / ISO mathematical definitions
Worked Example
A 98-day project sprint between August 15 and November 21 comprises exactly 14 full weeks, providing 70 business workdays for project deliverables.
Understanding Your Result
Evaluate your result in the context of your specific use case and relevant industry standards or reference values. A single calculated result is a starting point — test two to three scenarios to understand how sensitive the outcome is to your key input assumptions. For high-stakes decisions, verify your result against a secondary source or subject-matter expert before acting.
💡 Mathematical Rigor & Applied Context
- This calculator applies exact mathematical definitions following conventions established by NIST and international standards bodies.
- Rounding errors accumulate across multi-step calculations. For precision-critical work, maintain extra significant figures through all intermediate steps.
- Many mathematical concepts have multiple valid formulations — if a result seems unexpected, verify which convention or definition applies to your context.
- Dimensional analysis (unit tracking) is the fastest way to catch formula errors. Every term in an equation must have consistent, compatible units.
- Numerical methods used in digital calculators introduce floating-point precision limits (~15 significant digits for IEEE 754 double precision).
- For statistical and probabilistic calculations, always specify whether you are working with population parameters or sample statistics — formulas differ.
- Visualizing a problem geometrically or testing with known boundary values (zero, infinity, negative) reveals hidden errors faster than algebraic checking.
- When in doubt, validate your result against a simplified hand calculation or a published worked example from a textbook or standards document.
Results are for informational and educational purposes only. Always verify critical decisions with a qualified professional.