Calendar year reference guides
Compare, calculate and understand years across modern and historical calendar systems.
How to compare two years
Compare length, weekday alignment, weeks and dates across any two calendar years.
Leap years: rules and examples
Use the Gregorian rule to identify past and future leap years.
Years with the same calendar
Find years whose dates fall on the same weekdays.
Decades, centuries and millennia explained
Understand how named spans of years are numbered.
How to calculate years between two dates
Calculate complete elapsed years without off-by-one birthday errors.
How old is someone born in a given year?
Calculate age from a birth date or estimate it from a birth year.
The ISO week-numbering year
Why an ISO week-year can differ from the calendar year.
Common year vs leap year
Compare 365-day and 366-day calendars.
The Gregorian calendar reform
Why the modern leap-year system replaced the Julian calendar.
Year numbering: BCE, CE, BC and AD
Read historical years and understand the missing year zero.
Generation year ranges explained
Why generational labels have approximate, disputed boundaries.
Fiscal years and academic years
Understand annual periods that do not begin on January 1.
Historical calendar systems
A practical introduction to calendars used across eras and cultures.