Leap Year & Calendar Week Calculator
Verify leap year criteria, compute ISO-8601 week numbers, and track day of the year.
365 days in total. February has 28 days.
Complete Guide to Gregorian Leap Years, Solar Drift & ISO-8601 Calendar Weeks
Earth takes approximately 365.242189 days (365 days, 5 hours, 48 minutes, and 45 seconds) to orbit the Sun. Without periodic intercalary leap day corrections, solar equinoxes and seasons would drift by roughly 24 days every century. Modern Gregorian calendar mathematics solves this alignment through exact divisibility rules.
The 3-Tier Leap Year Algorithm
Every leap year must strictly satisfy the standard 3-tier algorithm introduced by Pope Gregory XIII in 1582:
- • Step 1: Is
year % 4 == 0? If NO → Common Year (365 days). - • Step 2: Is
year % 100 == 0? If YES → Proceed to Step 3. Otherwise → Leap Year (366 days). - • Step 3: Is
year % 400 == 0? If YES → Leap Year. If NO → Common Year.
ISO-8601 International Week System
Standard business weeks in logistics, corporate financial reporting, and manufacturing follow ISO-8601 rules:
• Weeks always start on Monday and end on Sunday.
• Week 01 is the week containing the first Thursday of January.
• Calendar years contain either 52 or 53 full ISO weeks.
Century Years & Leap Year Classification Benchmark
| Year | Divisible by 4 | Divisible by 100 | Divisible by 400 | Resulting Status |
|---|---|---|---|---|
| 1900 | Yes | Yes | No | Common Year (365 Days) |
| 2000 | Yes | Yes | Yes | Leap Year (366 Days) |
| 2024 | Yes | No | N/A | Leap Year (366 Days) |
| 2026 | No | No | N/A | Common Year (365 Days) |
| 2100 | Yes | Yes | No | Common Year (365 Days) |
| 2400 | Yes | Yes | Yes | Leap Year (366 Days) |
Frequently Asked Questions (FAQ)
Why was the year 2000 a leap year, but 1900 and 2100 are not?
Century years ending in '00' are special exceptions. Under Gregorian reform rules, century years must be evenly divisible by 400. 2000 divided by 400 equals exactly 5 (leap year), whereas 1900 and 2100 are not evenly divisible by 400.
When will the next leap year occur?
Following the 2024 leap year, the upcoming leap years are 2028, 2032, 2036, 2040, and 2044.
Can a date in January belong to ISO Week 52 or 53 of the previous year?
Yes! If January 1 falls on a Friday, Saturday, or Sunday, those initial days are considered part of Week 52 or Week 53 of the preceding calendar year according to ISO-8601 specifications.