Year Calculator
Calculate years between dates, add or subtract years from any date, convert years to other time units, and determine age in years with our comprehensive year calculator. Whether you need to find the exact number of years between two dates, add 5 years to a contract date, convert 2.5 years to months and days, or calculate someone's age, this tool handles all year-based date math with multiple conventions including calendar years, decimal years, and leap year handling.
Year Calculator
Date Range
Breakdown
Timeline Visualization
Quick Presets
📅 New Millennium to Today
2000-01-01 → Today
Years since Y2K
🗓️ Leap Year Edge Case
2020-02-29 → 2025-12-31
Tests leap year handling
➕ Add 10 Years
2015-06-01 + 10 years
Contract expiration date
🔄 Convert 2.5 Years
2.5 years to all units
Months, weeks, days
Calculation Methods
Decimal Years (Average Year)
Uses 365.2425 days per year (Gregorian calendar average accounting for leap years)
Actual/365 Convention
Common in bond calculations and some financial contexts
Actual/360 Convention
Used in many commercial loans and some bond markets
Calendar Years (Birthday Style)
Calendar years count completed anniversaries:
- Extract year, month, day from both dates
- Calculate: years = end_year - start_year
- If end month/day < start month/day, subtract 1 year
- This matches how we calculate age
Why Different Conventions?
- Calendar years: Natural for human ages, anniversaries, tenures
- Decimal years: Precise for scientific calculations, astronomy
- Actual/365: Some financial instruments, simplifies day count
- Actual/360: Banking convention, makes months "equal" at 30 days
How to Calculate Years Between Two Dates
Calculating the number of years between two dates depends on which convention you use. Each method serves different purposes and produces slightly different results.
Calendar Years Method
The calendar years method (also called birthday style or anniversary method) counts how many complete years have passed. This is how we calculate age: from your birthday to the day before your next birthday is 0 completed years; on your birthday, you've completed another year. For dates, subtract the start year from the end year, then adjust if the end month/day is earlier than the start month/day.
Decimal Years Method
Decimal years convert the exact time span to a fractional year value. Calculate the total days between dates, then divide by 365.2425 (the average year length in the Gregorian calendar). This accounts for leap years over time and provides the most astronomically accurate measurement. For example, 730 days = 730 ÷ 365.2425 ≈ 1.9986 years.
Financial Conventions
Financial markets use standardized day-count conventions. Actual/365 divides actual days by 365, while Actual/360 divides by 360 (treating each month as 30 days for simplicity). The 360-day convention makes some calculations easier and is common in commercial lending.
Calendar Years vs Decimal Years
Calendar Years
Calendar years measure time in whole completed years, just like calculating age. From January 15, 2020 to January 14, 2023 is 2 calendar years (not yet 3 because the third anniversary hasn't arrived). This method aligns with how we naturally think about years and is ideal for contracts, anniversaries, and age calculations.
Decimal Years
Decimal years express time as a precise fraction. From January 1, 2020 to July 1, 2020 is exactly 0.4986 years (182 days ÷ 365.2425). This method provides mathematical precision and continuous values, making it better for scientific calculations, interest accrual, or when you need exact fractional years.
When to Use Each
Use calendar years for legal contracts, employee tenure, age calculation, and any context where "turning" a year matters. Use decimal years for financial calculations, scientific measurements, statistical analysis, or when precision matters more than human convention.
Leap Years Explained
What is a Leap Year?
A leap year has 366 days instead of 365, with an extra day (February 29) added to keep our calendar synchronized with Earth's orbit around the sun. Earth's orbital period is approximately 365.2425 days—not exactly 365—so we need leap years to prevent calendar drift.
Leap Year Rules
A year is a leap year if it meets these criteria in order:
- If divisible by 4, it's a leap year
- UNLESS it's divisible by 100, then it's NOT a leap year
- UNLESS it's also divisible by 400, then it IS a leap year
Examples: 2020, 2024 are leap years (divisible by 4). 1900 was not (divisible by 100 but not 400). 2000 was (divisible by 400).
Impact on Year Calculations
Leap years affect calculations in several ways. Calendar years aren't affected—you still turn a year older on your birthday regardless of leap days. Decimal years automatically account for leap days in the total day count. When adding years to February 29, you must decide: move to February 28, move to March 1, or keep February 29 only when the target year is also a leap year.
Examples
Example 1: Age Calculation
Scenario: Someone born June 15, 1990, calculating age on December 31, 2025.
Calculation:
- Years: 2025 - 1990 = 35
- Since December 31 comes after June 15, they've completed 35 full years
- Months since last birthday: June 15 to December 31 = 6 months, 16 days
- Age: 35 years, 6 months, 16 days
Result: 35 calendar years old. In decimal years: ≈35.54 years.
Example 2: Contract Duration
Scenario: Contract from March 1, 2020 to February 28, 2025.
Calculation:
- Total days: 1,825 days (includes 2020 leap day)
- Calendar years: Exactly 5 years (Feb 28 to Feb 28)
- Decimal years: 1,825 ÷ 365.2425 = 4.9966 years
Result: 5 complete calendar years, or approximately 5.00 decimal years.
Example 3: Adding Years with Leap Day
Scenario: Add 5 years to February 29, 2020 (leap day).
Calculation (Feb 28 rule):
- Target year: 2020 + 5 = 2025
- 2025 is not a leap year (no Feb 29)
- With "move to Feb 28" rule: Result is February 28, 2025
Calculation (Mar 1 rule): Result would be March 1, 2025
Result: February 28 or March 1, 2025 depending on your rule preference.
Example 4: Unit Conversion
Scenario: Convert 2.5 years to various units.
Calculation:
- Months: 2.5 × 12 = 30 months
- Weeks: 2.5 × 52.1775 = 130.44 weeks
- Days: 2.5 × 365.2425 = 913.11 days
- Hours: 913.11 × 24 = 21,914.6 hours
Result: 2.5 years equals 30 months, approximately 130 weeks, 913 days, or 21,915 hours.
Common Mistakes
- Assuming all years are 365 days – Leap years have 366 days; use 365.2425 for average year length
- Not specifying the convention – "5 years" can mean different things: calendar years, decimal years, or financial years
- Off-by-one errors with inclusive dates – Clarify whether start and end dates are included or exclusive
- Forgetting leap day handling – Adding years to Feb 29 requires explicit rules for non-leap target years
- Confusing calendar and decimal years – 1.5 calendar years doesn't equal 1.5 decimal years precisely
- Timezone confusion – Date calculations can shift across midnight in different timezones
- Using 365 for financial calculations – Some financial contexts require Actual/360 (360-day year)
- Not accounting for completed years – From Jan 1, 2020 to Dec 31, 2020 is 0 completed years, not 1
- Assuming February always has 28 days – February has 29 days in leap years, affecting day counts
- Rounding decimal years incorrectly – 1.9986 years is not "2 years" in calendar year terms; it's still 1 completed year
Frequently Asked Questions
To calculate years between two dates, use one of these methods: (1) Calendar years (birthday style): Subtract the start year from the end year, then subtract 1 if the end month/day is earlier than the start month/day. This gives completed years. (2) Decimal years: Calculate total days between dates, then divide by 365.2425 (average year length). This gives precise fractional years. For example, from March 15, 2020 to March 14, 2025 is 4 complete calendar years (5 years minus 1 because we haven't reached the anniversary yet) or 4.9973 decimal years (1,825 days ÷ 365.2425).
A decimal year expresses time as a fractional year value, providing precise measurements rather than whole years. It's calculated by dividing the total number of days by 365.2425 (the average length of a year in the Gregorian calendar, accounting for leap years). For example, 6 months (approximately 182.62 days) equals about 0.5 decimal years, and 18 months equals 1.5 decimal years. Decimal years are used in scientific calculations, astronomy, financial analysis, and any context requiring mathematical precision rather than human-readable whole years.
Calendar years count completed whole years (like age), while decimal years provide precise fractional values. Calendar years: From your birthday to the day before your next birthday is 0 years; on your birthday you turn 1 year older. This is discrete and anniversary-based. Decimal years: Continuous measurement where time is expressed as a decimal (1.5 years, 2.75 years). Example: From Jan 1, 2020 to July 1, 2020 is 0 calendar years (not yet completed one year) but 0.4986 decimal years (182 days ÷ 365.2425). Use calendar years for legal contracts and ages; use decimal years for mathematical precision.
Leap years follow a three-rule system: (1) If a year is divisible by 4, it's a leap year. (2) Exception: If divisible by 100, it's NOT a leap year. (3) Exception to exception: If divisible by 400, it IS a leap year. Examples: 2024 is a leap year (÷4). 1900 was not (÷100 but not ÷400). 2000 was a leap year (÷400). This system keeps our calendar synchronized with Earth's 365.2425-day orbital period. The rule adds leap days approximately every 4 years (365.25 days), removes some with the 100-year rule, then adds back with the 400-year rule, averaging exactly 365.2425 days per year.
To add years to a date: (1) Add the number of years to the year component. (2) Keep the same month and day. (3) Handle February 29 edge case: if the start date is Feb 29 and the target year isn't a leap year, choose a rule: move to Feb 28 (most common), move to Mar 1, or only use Feb 29 when valid. Example: Jan 15, 2020 + 5 years = Jan 15, 2025. Edge case: Feb 29, 2020 + 1 year = Feb 28 or Mar 1, 2021 (2021 isn't a leap year). Most calculators default to Feb 28. The same logic applies to subtracting years.
The average year length in the Gregorian calendar is 365.2425 days. This accounts for leap years: most years have 365 days, leap years have 366, and the leap year rules (divisible by 4, except centuries unless divisible by 400) average out to exactly 365.2425 days per year over long periods. This extremely close approximation to Earth's orbital period (365.242189 days) means our calendar drifts only about 1 day every 3,236 years. For quick calculations, 365.25 is often used, but 365.2425 is more accurate for scientific work.
Actual/360 and Actual/365 are day-count conventions used in finance. Actual/360: Divides actual days by 360, treating each month as 30 days. Common in commercial loans, money markets, and some bonds. Results in slightly higher interest accrual. Actual/365: Divides actual days by 365, closer to true calendar time. Used in some bonds and financial instruments. Both use "actual" days in the numerator but differ in the denominator. Example: 90 days of interest at 6% annual rate: Actual/360 = (90/360) × 6% = 1.5%; Actual/365 = (90/365) × 6% = 1.479%. The difference matters for large principal amounts or long periods.
Calculate age using the calendar years method: (1) Subtract birth year from current year. (2) If current month/day is before birth month/day, subtract 1 (you haven't had your birthday yet this year). (3) For precise age, calculate remaining months and days. Example: Born June 15, 1990, calculating on March 10, 2025: 2025 - 1990 = 35. Since March 10 is before June 15, subtract 1 = 34 years old. More precisely: 34 years, 8 months, 24 days old (from June 15, 2024 to March 10, 2025). You turn 35 on June 15, 2025.
We have leap years because Earth's orbit around the sun takes approximately 365.2425 days, not exactly 365. If we used only 365-day years, seasons would drift by about 0.2425 days (nearly 6 hours) per year. After 100 years, we'd be off by 24 days—summer would arrive in winter! Leap years add an extra day every ~4 years to compensate. The three-part rule (divisible by 4, except centuries unless divisible by 400) fine-tunes this to match Earth's actual orbital period almost exactly, keeping our calendar synchronized with astronomical seasons over millennia.
There are approximately 52.1775 weeks in an average year. Calculation: 365.2425 days ÷ 7 days per week = 52.1775 weeks. In practical terms, most years have 52 full weeks plus 1 day (365 ÷ 7 = 52 remainder 1), while leap years have 52 full weeks plus 2 days (366 ÷ 7 = 52 remainder 2). This is why the same date falls on a different day of the week each year—it shifts forward by 1 day in regular years and 2 days in leap years. For business planning, "52 weeks" is commonly used, acknowledging there are actually slightly more.
Yes, in ISO week date system, some years have 53 weeks. ISO weeks run Monday to Sunday, and week 1 is the first week with at least 4 days in the new year. A year has 53 ISO weeks when January 1 falls on a Thursday, or when it falls on a Wednesday in a leap year. This happens approximately every 5-6 years. Examples: 2015, 2020, and 2026 have 53 ISO weeks. In standard calendar counting, all years have 52 full weeks plus 1-2 extra days, but in business and accounting contexts using ISO weeks, these are grouped into a 53rd week.
When adding years to February 29 (leap day) where the target year is not a leap year, you must choose a convention: (1) Move to February 28 (most common): Feb 29, 2020 + 1 year = Feb 28, 2021. This keeps you in February and is closest to the original date. (2) Move to March 1: Maintains the same number of days from year-end. (3) Keep Feb 29 only when valid: Feb 29, 2020 + 4 years = Feb 29, 2024 (valid leap year), but + 1 year requires choosing option 1 or 2. Financial systems, databases, and legal contracts specify which rule to use. Excel and most programming languages default to February 28.