Section 7.13. Obtaining the Time Zone

7.12. Detecting Leap Years

The Date class has two class methods julian_leap? and gregorian_leap?; only the latter is of use in recent years. It also has a method leap?, which is an alias for the gregorian_leap? method.

require "date" flag1 = Date.julian_leap? 1700     # true flag2 = Date.gregorian_leap? 1700  # false flag3 = Date.leap? 1700            # false

Every child knows the first rule for leap years: The year number must be divisible by four. Fewer people know the second rule, that the year number must not be divisible by 100; and fewer still know the exception, that the year can be divisible by 400. In other words: A century year is a leap year only if it is divisible by 400, so 1900 was not a leap year, but 2000 was. (This adjustment is necessary because a year is not exactly 365.25 days, but a little less, approximately 365.2422 days.)

The Time class does not have a method like this, but if we needed one, it would be simple to create.

class Time   def Time.leap? year     if year % 400 == 0       true     elsif year % 100 == 0       false     elsif year % 4 == 0       true     else       false   end end

I've written this to make the algorithm explicit; an easier implementation, of course, would be simply to call the Date.leap? method from this one. I implement this as a class method by analogy with the Date class methods. It could also be implemented as an instance method.