<=>(p1) public

Compares the two dates and returns -1, zero, 1 or nil. The other should be a date object or a numeric value as an astronomical Julian day number.

Date.new(2001,2,3) <=> Date.new(2001,2,4) #=> -1
Date.new(2001,2,3) <=> Date.new(2001,2,3) #=> 0
Date.new(2001,2,3) <=> Date.new(2001,2,2) #=> 1
Date.new(2001,2,3) <=> Object.new         #=> nil
Date.new(2001,2,3) <=> Rational(4903887,2)#=> 0

See also Comparable.

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March 12, 2009
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Comparing Date with Numeric in mixed sort

While:

Date#<=>(other)

can accept a Numeric object as other, the reverse is not true:

Numeric#<=>(other)

cannot accept a Date object as other.

So if you are sorting a list containing a mix of dates and numbers, you can get different results depending on the starting order!

a = Date.parse("2008-01-01")
b = Date.parse("2009-10-22")
c = Date.parse("2005-01-04")
d = 0

[a,b,c,d].sort #=> [0, Tue, 04 Jan 2005, Tue, 01 Jan 2008, Thu, 22 Oct 2009]

[b,c,d,a].sort #=> ArgumentError: comparison of Fixnum with Date failed