Date Calculator Guide: Days Between Dates, Add/Subtract Days & Age (2026)

The formula is simple: subtract the earlier date from the later date. But edge cases — leap years, month lengths, timezone differences — make manual calculation error-prone.

Quick Reference: Days in Each Month

Leap Year Rule

A year is a leap year if: divisible by 4, EXCEPT centuries (divisible by 100) UNLESS also divisible by 400. So 2024 = leap, 2100 = not leap, 2000 = leap. This adds 1 day to February.

Need to know what date falls 90 days from today? Or 45 business days before a deadline? Adding or subtracting days is the most common date calculation.

Common Use Cases

Business days (working days) exclude Saturdays and Sundays. For India-specific calculations, you may also need to exclude gazetted holidays.

Business Days vs Calendar Days

Indian Public Holidays (2026 — Central Government)

India has approximately 15-17 gazetted holidays per year. Banks observe additional holidays (RBI holiday list). States add regional holidays. For precise business day calculations, always check the applicable holiday calendar for your state and sector.

Ever wondered exactly how old you are — not just in years, but in days, weeks, or even hours? The date calculator makes this easy.

Age Breakdown Example: Born 15 August 1995 (as of 8 April 2026)

Age Calculation for Indian Government Applications

Many Indian government forms (UPSC, SSC, Railways, bank exams) require exact age calculation as of a specific cutoff date. The age calculator gives you the precise years-months-days breakdown needed for eligibility verification. Age relaxation rules (SC/ST: +5 years, OBC: +3 years) apply on top of the base age.

Financial Year Calculations

Indian Financial Year runs April 1 to March 31. Key dates:

• ITR filing deadline — 31 July (individuals), 31 October (audit cases)
• Advance tax instalments — 15 Jun, 15 Sep, 15 Dec, 15 Mar
• TDS return filing — Within 30 days of quarter end

Pregnancy Due Date (Naegele's Rule)

Due date = First day of last menstrual period (LMP) + 280 days (40 weeks). This is the most common obstetric calculation and the date calculator handles it precisely.

Retirement Date Calculation

Central government retirement age is 60 (superannuation on the afternoon of the last day of the month in which you turn 60). State governments vary: 58-62. Calculate exact retirement date from your date of birth.

Notice Period Calculations

Date format confusion causes real errors — especially in international business. "04/08/2026" means April 8 in the US but August 4 in India/UK.

Unix Timestamp

Unix timestamp = seconds since 1 January 1970 00:00:00 UTC. Example: 8 April 2026 00:00:00 UTC = 1775692800. Days between two timestamps: (t2 - t1) / 86400.

Common Pitfalls

• Timezone bugs — Midnight in IST is 18:30 previous day in UTC. Always store dates in UTC and convert for display.
• Daylight Saving Time — Adding 24 hours is not always adding 1 day. Use date libraries (date-fns, dayjs, Luxon) instead of raw arithmetic.
• Month indexing — JavaScript months are 0-indexed (January = 0). This causes off-by-one errors.
• Leap second — Not relevant for most apps, but explains why some timestamps have 61 seconds in a minute.

Useful Formulas

// Days between two dates (JavaScript)
const diffDays = Math.ceil((date2 - date1) / (1000 * 60 * 60 * 24));

// Add N days to a date
const newDate = new Date(date);
newDate.setDate(newDate.getDate() + N);

// Is leap year?
const isLeap = (y) => (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0;