Date Calculator Complete Guide 2026

"What date is 100 days from now?" "How many days until the exam?" "What's my exact age?"

Date calculations seem simple, but they're easy to get wrong by hand due to leap years and varying days in each month. This guide covers everything about date calculations.

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Date Calculation Basics

Days in Each Month

Knuckle Memory Method

When making a fist:

• Knuckle bumps: 31 days (Jan, Mar, May, Jul, Aug, Oct, Dec)
• Valleys between: 30 days or 28/29 days

Leap Year Rules

Leap year conditions:
1. Divisible by 4 = leap year
2. But divisible by 100 = common year
3. But divisible by 400 = leap year

Examples:
2024: Divisible by 4 → Leap year ✓
2100: Divisible by 100 → Common year
2000: Divisible by 400 → Leap year ✓

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D-day Calculations

Basic Principle

D-day = Target date - Today
D+n = n days after target date
D-n = n days before target date

Manual Calculation Method

Example: March 15, 2026 → June 20, 2026

Remaining days in March: 31 - 15 = 16 days
All of April: 30 days
All of May: 31 days
Days passed in June: 20 days

Total: 16 + 30 + 31 + 20 = 97 days

D-day Notation

Common D-day Examples

Couple's 100th day: Dating start + 99 days = 100th day
1st anniversary: Dating start + 365 days (mind leap years)
Due date: Last period + 280 days

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Calculating Duration Between Two Dates

Counting Days

2026-01-15 ~ 2026-03-20

Method 1: Date difference
Remaining days in Jan: 31 - 15 = 16
All of February: 28 (2026 is not a leap year)
Days in March: 20
Total: 16 + 28 + 20 = 64 days

Method 2: Use a date calculator (recommended)

Counting Weeks

Days ÷ 7 = Weeks
64 days ÷ 7 = 9 weeks and 1 day

Counting Months

Month counting varies by definition:

2026-01-15 ~ 2026-04-15

Method 1: Simple month count
4 - 1 = 3 months

Method 2: Day-based
90 days ÷ 30 = 3 months

Method 3: Exact month boundaries
1/15 → 2/15 → 3/15 → 4/15 = Exactly 3 months

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Age Calculations

Traditional vs Exact Age

Most countries use exact age (international standard)

Exact Age Calculation

function calculateAge(birthDate) {
  const today = new Date();
  let age = today.getFullYear() - birthDate.getFullYear();

  // Subtract 1 if birthday hasn't occurred yet
  const monthDiff = today.getMonth() - birthDate.getMonth();
  if (monthDiff < 0 ||
      (monthDiff === 0 && today.getDate() < birthDate.getDate())) {
    age--;
  }

  return age;
}

Age Calculation Examples

Birthdate: June 15, 2000
Today: February 21, 2026

Exact age: 25 years old (before birthday)

Today: July 1, 2026
Exact age: 26 years old (after birthday)

Special Cases

Born on February 29 (Leap Day):

• In common years, calculated as Feb 28 or March 1
• Legally, usually turns a year older at midnight on Feb 28

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Business Day Calculations

Business Day Counting Principle

Total days - Weekends - Holidays = Business days

Example: March 2026 (31 days)
Weekends: 8 days (4 Saturdays + 4 Sundays)
Holidays: 0 days
Business days: 31 - 8 - 0 = 23 days

2026 US Federal Holidays

Business Day Shipping

"Delivery within 3 business days"

Friday order → Mon, Tue, Wed (arrives Wednesday)
Wednesday order → Thu, Fri, Mon (arrives Monday)

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Time Zone Calculations

Major Time Zones

Daylight Saving Time (DST)

US/Europe applies DST:

• 2nd Sunday of March ~ 1st Sunday of November (US)
• Clocks move forward 1 hour

During DST:
New York: UTC-4 (normally UTC-5)
Los Angeles: UTC-7 (normally UTC-8)

Time Conversion Example

New York 3 PM → Tokyo time?

Standard time: 3 PM + 14 = 5 AM (next day)
During DST: 3 PM + 13 = 4 AM (next day)

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Practical Date Calculations

Week Number Calculation

ISO standard: Week containing Thursday belongs to that year

Jan 1 is Thursday: Week 1 starts
Jan 1 is Fri/Sat/Sun: Last week of previous year

Quarter Calculation

Half-year Calculation

First half: Jan-Jun (181 days, 182 in leap year)
Second half: Jul-Dec (184 days)

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Date Calculations for Developers

Unix Timestamp

Unix Timestamp: Seconds since 1970-01-01 00:00:00 UTC

Current time → Timestamp
new Date().getTime() / 1000

Timestamp → Date
new Date(timestamp * 1000)

ISO 8601 Format

Standard format: YYYY-MM-DDTHH:mm:ss.sssZ

Example: 2026-02-21T15:30:00.000-05:00
         2026-02-21T20:30:00.000Z (UTC)

Date Libraries

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Special Date Calculations

Pregnancy Weeks

Pregnancy weeks = (Today - Last period date) ÷ 7

Due date = Last period date + 280 days (40 weeks)

Naegele's rule:
Due date = Last period first day - 3 months + 7 days + 1 year

Lunar Calendar Conversion

Lunar calendar is based on moon cycles (~29.5 days):

• Varies from solar calendar
• Has leap months (7 times in 19 years)
• Requires astronomical data for accurate conversion

Chinese New Year 2026: Lunar Jan 1 = Solar Feb 17
Mid-Autumn 2026: Lunar Aug 15 = Solar Oct 6

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FAQ

Q1: What's the difference between D-day and D+day?

A:

• D-30: 30 days remaining (future)
• D-day: The day itself
• D+30: 30 days passed (past)

Q2: What exactly is "3 months later"?

A: Depends on context:

• Legal period: Same date (1/15 → 4/15)
• Day-based: 90 days later
• Should be specified in contracts

Q3: When do people born on Feb 29 celebrate birthdays?

A:

• Legally, turns a year older at midnight on Feb 28 (varies by country)
• In common years, celebrate on Feb 28 or March 1
• Different countries have different rules

Q4: Is the 100th day anniversary the 100th day or 99 days later?

A: Usually the 100th day including the first day:

• January 1 start → April 10 is the 100th day

1/1 = Day 1
4/10 = Day 100 (99 days later)

Q5: What does "business days" mean?

A: Number of days excluding weekends and holidays. Excludes Sat/Sun/legal holidays.

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Conclusion

Key date calculation summary:

1. D-day: Days remaining until target date
2. Exact age: Precise age based on birthday
3. Business days: Total days - Weekends - Holidays
4. Time zones: Calculate based on UTC offset
5. Leap year: Divisible by 4 is leap year (100/400 exceptions)

Use tools for complex calculations!

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