litany against fear

Calculating Age in Rails

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
  Date.today.year - birthday.year
end

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 Date.today, but not after.

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

def age
  ((Date.today - birthday) / 365).floor
end

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.