How do you do relative time in Rails?

Sounds like you're looking for the time_ago_in_words method (or distance_of_time_in_words), from ActiveSupport. Call it like this:

<%= time_ago_in_words(timestamp) %>

I've written this, but have to check the existing methods mentioned to see if they are better.

module PrettyDate
  def to_pretty
    a = (Time.now-self).to_i

    case a
      when 0 then 'just now'
      when 1 then 'a second ago'
      when 2..59 then a.to_s+' seconds ago' 
      when 60..119 then 'a minute ago' #120 = 2 minutes
      when 120..3540 then (a/60).to_i.to_s+' minutes ago'
      when 3541..7100 then 'an hour ago' # 3600 = 1 hour
      when 7101..82800 then ((a+99)/3600).to_i.to_s+' hours ago' 
      when 82801..172000 then 'a day ago' # 86400 = 1 day
      when 172001..518400 then ((a+800)/(60*60*24)).to_i.to_s+' days ago'
      when 518400..1036800 then 'a week ago'
      else ((a+180000)/(60*60*24*7)).to_i.to_s+' weeks ago'
    end
  end
end

Time.send :include, PrettyDate

What about

30.seconds.ago
2.days.ago

Or something else you were shooting for?

You can use the arithmetic operators to do relative time.

Time.now - 2.days 

Will give you 2 days ago.

Something like this would work.

def relative_time(start_time)
  diff_seconds = Time.now - start_time
  case diff_seconds
    when 0 .. 59
      puts "#{diff_seconds} seconds ago"
    when 60 .. (3600-1)
      puts "#{diff_seconds/60} minutes ago"
    when 3600 .. (3600*24-1)
      puts "#{diff_seconds/3600} hours ago"
    when (3600*24) .. (3600*24*30) 
      puts "#{diff_seconds/(3600*24)} days ago"
    else
      puts start_time.strftime("%m/%d/%Y")
  end
end

Another approach is to unload some logic from the backend and maek the browser do the job by using Javascript plugins such as:

jQuery time ago or its Rails Gem adaptation