A year is one Earth orbit — about 365.25 days. The extra 0.25 is why we add a leap day every four years.
Quick check
Why is a year about 365.25 days, not a whole number?
The leftover quarter-day
Each year we round down to 365 days, leaving ≈0.25 day unused. After 4 years that's almost a full day — so we add Feb 29 in a leap year to catch up. Tiny corrections (skip the leap day in most century years) keep it precise over centuries.
Year sizes
- Common year — 365 days
- Leap year — 366 days
- 1 year = 12 months = 52 weeks (and a bit)
- More precisely: 365.2422 days (the 'tropical year')
A *light-year* is not a unit of time — it's a distance: how far light travels in one year, about 9.46 trillion km.