How to get the number of days in a given month in Ruby, accounting for year?

I'm sure there's a good simple elegant one-liner in Ruby to give you the number of days in a given month, accounting for year, such as "February 1997". What is it?

If you're working in Rails, chances are you'll get hamstrung eventually if you switch among Time, Date, and DateTime, especially when it comes to dealing with UTC/time zones, daylight savings, and the like. My experience has been it's best to use Time, and stick with it everywhere.

So, assuming you're using Rails's Time class, there are two good options, depending on context:

1. If you have a month m and year y, use the class method on Time:

    days = Time.days_in_month(m, y)
   

2. If you have a Time object t, cleaner to ask the day number of the last day of the month:

    days = t.end_of_month.day
   

require 'date'

def days_in_month(year, month)
  Date.new(year, month, -1).day
end

# print number of days in February 2012
puts days_in_month(2012, 2)

This is the implementation from ActiveSupport (a little adapted):

COMMON_YEAR_DAYS_IN_MONTH = [nil, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

def days_in_month(month, year = Time.now.year)
   return 29 if month == 2 && Date.gregorian_leap?(year)
   COMMON_YEAR_DAYS_IN_MONTH[month]
end

How about:

require 'date'

def days_in_month(year, month)
  (Date.new(year, 12, 31) << (12-month)).day
end

# print number of days in Feburary 2009
puts days_in_month(2009, 2)

You may also want to look at Time::days_in_month in Ruby on Rails.

In Rails project for current date

Time.days_in_month(Time.now.month, Time.now.year)

For any date t which is instance of Time

Time.days_in_month(t.month, t.year)

or

t.end_of_month.day

.
If you have UTC seconds, you need to get an instance of Time first

Time.at(seconds).end_of_month.day

Use Time.days_in_month(month) where month = 1 for January, 2 for Feb, etc.

For a given Date object I feel like the easiest is:

Date.today.all_month.count

revise the input for other format

def days_in_a_month(date = "2012-02-01")
  date.to_date.end_of_month.day
end

as of rails 3.2... there's a built in version: http://api.rubyonrails.org/classes/Time.html#method-c-days_in_month

(alas, it shows up after this answer, which takes folks on a long hike)

Time.now.end_of_month.day - for current month

Date.parse("2014-07-01").end_of_month.day - use date of first day in month.

Depends on ActiveSupport

A simple way using Date:

def days_of_month(month, year)
  Date.new(year, month, -1).day
end

I think it's the simplest way to get it

def get_number_of_days(date = Date.today)
  Date.new(date.year, date.month, -1).mday
end

Since the time is irrelevant for this purpose then you just need a date object:

[*Date.today.all_month].size