litany against fear

coding with spice ¤ by nick quaranto

Calculating Age in Rails

published 13 Oct 2008

You’d think that this would be easy, but for some reason it wasn’t, at least for me. Let’s say you keep track of a User’s birthday with a date field. Great! Let’s show the user’s age.

def age - birthday.year

Done! Right? WRONG. Worked fine for some users, until one of my coworkers asked me…hey, you’re not 21 yet…how’d you magically gain a year on your profile? Crap. Obviously this will work for everyone’s whose birthday is BEFORE, but not after.

So, we need a more exact method of calculating the age:

def age
  (( - birthday) / 365).floor

So what this method does instead is use the Rational value given by subtracting two Dates, and divides it by the number of days in a year. Since that number is still a Rational, calling floor on it will round it down to the nearest Fixnum, giving us a (slightly more) precise account of this user’s age. EDIT: But this is still inaccurate!

Yeah, I thought it was easy too. Perhaps this shows that I need to test my app a little more thoroughly! It also would be nice to save the age in an instance level variable, but right now I don’t use the age more than once on a page so it doesn’t matter.

EDIT: It looks like my method is still not sufficient for leap years! The comments have posted MANY great solutions. My method is fine for a general estimation, but the comments have many solutions for that deal with greater accuracy. I’m honestly not sure which is best, so choose carefully when you’re developing your app.

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