How can I calculate the number of weeks since a given date in Ruby?

Question

Objective

I am trying to calculate the distance in weeks since a given date without jumping through hoops. I'd prefer to do it in plain Ruby, but ActiveSupport is certainly an acceptable alternative.

My Code

I wrote the following, which seems to work but looks like the long way around to me.

require 'date'

DAYS_IN_WEEK = 7.0 

def weeks_since date_string
  date  = Date.parse date_string
  days  = Date.today - date
  weeks = days / DAYS_IN_WEEK
  weeks.round 2
end

weeks_since '2015-06-15'
#=> 32.57

ActiveSupport's #weeks_since takes a number of weeks as its argument, so it doesn't fit this use case. Ruby's Date class doesn't seem to have anything relevant, either.

Alternatives?

Is there a better built-in solution or well-known algorithm for calculating the number of weeks separating a pair of dates? I'm not trying to code-golf this, as readability trumps brevity, but simply to learn whether Ruby natively supports the type of date arithmetic I've coded by hand.

Answer 1

Might be easier to convert the dates to time and then divide the time difference by a week. You can round it however you want or ceil.

def weeks_since(date_string)
  time_in_past = Date.parse(date_string).to_time
  now = Time.now
  time_difference = now - time_in_past
  (time_difference / 1.week).round(2)
end

Answer 2

in_weeks (Rails 6.1+)

Rails 6.1 introduces new ActiveSupport::Duration conversion methods like in_seconds, in_minutes, in_hours, in_days, in_weeks, in_months, and in_years.

As a result, now, your problem can be solved as:

date_1 = Time.parse('2020-10-18 00:00:00 UTC')
date_2 = Time.parse('2020-08-13 03:35:38 UTC') 

(date_2 - date_1).seconds.in_weeks.to_i.abs
# => 9

Here is a link to the corresponding PR.