Date Difference – Kalkulačka
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- Zadejte Start Date
- Zadejte End Date
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How Date Difference Is Calculated
Calculating the exact difference between two dates requires careful handling of varying month lengths, leap years, and calendar rules. The process:
- Convert both dates to a common unit (e.g., total days from a reference point like January 1, 1970)
- Subtract the earlier date's day count from the later date's day count
- Convert the result back into years, months, weeks, and days
Example calculation: From March 15, 2020 to November 8, 2025:
- Total days: 2,064 days
- = 5 years, 7 months, 24 days
- = 294 weeks and 6 days
- = 2,064 days
Why this is tricky manually: Months have 28, 29, 30, or 31 days. A month-based calculation from March 15 to April 15 is "1 month" regardless of whether that span is 31 or 28 days. Our calculator handles all these edge cases automatically.
Leap Year Rules: When February Has 29 Days
Correct date calculations require understanding leap years. The Gregorian calendar rules are:
- A year divisible by 4 is a leap year (e.g., 2024 ✓)
- Exception: Century years (divisible by 100) are NOT leap years (e.g., 1900 ✗)
- Exception to the exception: Century years divisible by 400 ARE leap years (e.g., 2000 ✓)
So: 2000 = leap year ✓, 1900 = not a leap year ✗, 2100 = will NOT be a leap year ✗.
| Year | Divisible by 4? | Century year? | Divisible by 400? | Leap Year? |
|---|---|---|---|---|
| 2024 | Yes | No | — | ✓ Yes |
| 2025 | No | No | — | ✗ No |
| 2100 | Yes | Yes | No | ✗ No |
| 2000 | Yes | Yes | Yes | ✓ Yes |
| 1900 | Yes | Yes | No | ✗ No |
Over 400 years, there are exactly 97 leap years, making the average year 365.2425 days long — closely matching the actual solar year of 365.2422 days.
Business Days vs Calendar Days
For many practical purposes (contracts, deadlines, project planning), you need business days (working days) rather than calendar days:
Business days = Calendar days − Weekends − Public holidays
In a standard 5-day workweek, a 30-calendar-day period has approximately 22 business days (subtracting ~8 weekend days, minus any holidays).
| Calendar Days | Approximate Business Days (US) |
|---|---|
| 7 days (1 week) | 5 business days |
| 14 days (2 weeks) | 10 business days |
| 30 days (1 month) | ~22 business days |
| 90 days (1 quarter) | ~65 business days |
| 365 days (1 year) | ~261 business days |
Important: US federal holidays reduce this further. There are 11 federal holidays, bringing the actual US business days per year to approximately 250. Different countries have different public holidays — UK has 8 bank holidays, Germany has 9–13 (varies by state), France has 11.
Practical Uses for Date Difference Calculation
Knowing the exact number of days between dates is essential in many real-world situations:
- Legal and financial deadlines: Contracts often specify payment within "30 days" or "net 60" — precision matters for avoid default or missed deadlines. Many states calculate statutes of limitations to the day.
- Loan interest calculation: Many loans accrue interest daily. Knowing the exact number of days helps verify interest charges. A 1-day difference on a $500,000 commercial loan can be $100+.
- Project management: Project milestones are measured in calendar or business days. A 90-day software project starting March 1 ends May 30 (calendar) or about June 2 when excluding weekends.
- Medical dosing and treatment schedules: Antibiotic courses (7, 10, 14 days), pregnancy milestones (weeks since last menstrual period), and cancer treatment cycles all require precise date tracking.
- Anniversaries and events: Counting down days to weddings, birthdays, or retirements.
Age, Duration, and Date Arithmetic: Common Calculations
Here are common date math scenarios with worked examples:
| Scenario | Start | End | Result |
|---|---|---|---|
| Days in 2024 (leap year) | Jan 1, 2024 | Dec 31, 2024 | 366 days |
| Days until retirement (age 65 from DOB Mar 15, 1970) | Mar 11, 2026 | Mar 15, 2035 | 3,292 days |
| 90-day contract deadline | Jan 15, 2026 | + 90 days | April 15, 2026 |
| Weeks of pregnancy (LMP Nov 1) | Nov 1, 2025 | Mar 11, 2026 | 19 weeks, 0 days |
| Investment holding period (capital gains) | Oct 5, 2024 | Oct 6, 2025 | 366 days (long-term threshold met) |
US Capital Gains tax note: Long-term capital gains rates apply to assets held for more than 365 days. The difference between 364 and 366 days can change your tax rate from up to 37% (short-term, ordinary income rates) to 0%, 15%, or 20% (long-term rates). Exact date arithmetic matters.
Julian Day Number and Date Arithmetic Internals
Behind the scenes, date arithmetic software (including our calculator) works by converting each date to an absolute day count, performing arithmetic, then converting back. The most common standard is the Julian Day Number (JDN), which counts days from January 1, 4713 BC.
More commonly in computing, dates are stored as the number of days (or milliseconds) since a reference date called the Unix epoch: January 1, 1970 at 00:00 UTC.
- January 1, 2000 = Day 10,957 since the Unix epoch
- January 1, 2026 = Day 20,454 since the Unix epoch
This is why date subtraction is simple in programming: March 11, 2026 (day 20,527) minus March 15, 2020 (day 18,336) = 2,191 days.
Why the Gregorian calendar is awkward: Unlike metric units where everything divides by 10, calendar arithmetic involves irregular months (28–31 days), variable leap years, and base-60 time. No wonder people reach for a calculator!
Frequently Asked Questions
How many days are between two dates?
Enter both dates in our calculator for an instant count. To do it manually, convert each date to day-of-year and subtract, accounting for different year lengths. A leap year has 366 days; a standard year has 365. Our calculator handles all edge cases automatically.
How are leap years calculated?
A year is a leap year if: (1) divisible by 4, AND (2) if it is a century year, also divisible by 400. So 2000 was a leap year (divisible by 400), 1900 was not (divisible by 100 but not 400), and 2024 is a leap year. The next century leap year is 2400.
How do I calculate business days between two dates?
Count calendar days, then subtract the number of Saturdays and Sundays in that range, and subtract any public holidays. Approximately: calendar days × 5/7 gives a rough business day count. In the US, there are roughly 261 weekdays per year minus 11 federal holidays = 250 business days.
How many days until a specific date?
Subtract today's date from the target date. Our calculator does this automatically. For example, if today is March 11, 2026, there are 20 days until March 31, 2026 — and 356 days until March 2, 2027.
What is a fortnight?
A fortnight is exactly 14 days (2 weeks). The word comes from Old English 'fēowertyne niht' (fourteen nights). It is commonly used in British English for payroll (paid fortnightly) and some contract terms.
What is the difference between calendar days and business days?
Calendar days count all 7 days of the week. Business days (working days) only count Monday through Friday, excluding weekends and public holidays. A '30-day' deadline usually means 30 calendar days; a '20 business days' deadline means 4 work weeks.
How many weeks are in a year?
A standard year has 52 weeks and 1 day (365 days ÷ 7 = 52.14). A leap year has 52 weeks and 2 days (366 ÷ 7 = 52.28). This is why the same date shifts forward by 1 day of the week each year, and by 2 days after a leap year.
Why does the 90-day period matter for taxes and legal deadlines?
Many legal and financial deadlines are expressed as specific day counts. The IRS has 90-day filing deadlines for certain notices. US capital gains become 'long-term' (lower tax rate) after 365 days of holding. Insurance policies may have 30-day notice requirements. Exact calendar day calculation prevents costly mistakes.
💡 Věděli jste?
- The Gregorian calendar was introduced by Pope Gregory XIII in October 1582 to correct a 10-day drift that had accumulated in the Julian calendar since 325 AD.
- Leap years occur every 4 years — except for century years not divisible by 400. That's why 1900 was not a leap year, but 2000 was.
- The longest possible gap between two Friday the 13ths in any given year is exactly 14 months.
Naposledy aktualizováno: March 2026