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

Question

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?

score 137 · Answer 1 · edited Sep 02 '12 at 15:07

137

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:

If you have a month m and year y, use the class method on Time:
```
days = Time.days_in_month(m, y)
```
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
```

edited Sep 02 '12 at 15:07

Andrew Marshall

95,083
20
220
214

answered May 17 '12 at 17:40

mkosma

1,487
2
9
5

8

Thanks! but not really -- OP asked about ruby, not rails. I'd be interested to know how many folks are working in Ruby Off Rails, though. – mkosma Oct 17 '12 at 22:19
14

I use ruby outside rails and this answer would be no good for me. – SteveRawlinson Sep 06 '13 at 14:37
1

@SteveRawlingson Get your career on track then! :P – Starkers Feb 22 '14 at 20:44
2

I'm implementing a gem where this is needed, and I cannot assume the user would use the gem inside Rails. So I really think that, although cleaner, this should not be the accepted answer. It's very nice to have it here in context as well, cause lots of people would prefer this option when in Rails. – Ernesto Oct 02 '14 at 15:04
Working on Ruby project do not forbid you using activesupport gem. Source : https://github.com/rails/rails/tree/master/activesupport – OmniBus Feb 05 '15 at 06:56
I don't use Rails, but generally I find the time stuff from ActiveSupport is worth cherry-picking into any project that deals with dates and times like this. – Andrew Feb 09 '17 at 22:21

score 75 · Answer 2 · edited Jun 30 '17 at 01:21

75

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)

edited Jun 30 '17 at 01:21

coreyward

77,547
20
137
166

answered Nov 15 '12 at 15:23

crantok

1,333
11
18

btw I didn't need to require Date in my ruby only program. – Michael Durrant Oct 18 '13 at 13:23
4

You only didn't need to require date because it has already been required elsewhere. – Kris May 18 '18 at 08:46

score 27 · Accepted Answer · answered Sep 29 '09 at 00:09

27

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

answered Sep 29 '09 at 00:09

andre-r

2,685
19
23

1

Btw, originally implemented in module Time. – andre-r Sep 29 '09 at 00:13
9

super dated at this point. look at some of the other answers. – drewish May 20 '14 at 17:55
1

If you are using rails, @mkosma has a simpler answer. – Jeff Davenport Jan 06 '17 at 23:48

score 24 · Answer 4 · answered Sep 29 '09 at 00:14

24

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.

answered Sep 29 '09 at 00:14

1

Could you explain what make this def? Because I like but I want to understand – Bruce_Warrior Oct 15 '12 at 01:25
1

@Mike Wade, this assumes we know December has 31 days. What about if we don't even know that? – JWL Nov 02 '12 at 07:08
5

December will always have 31 days. – Alex Neth Nov 08 '12 at 20:38
1

@nemesisfixx, you can work off Jan 1 instead of Dec 31. ((Date.new(year+1, 1, 1) - 1.day) << (12-month)).day – Isaac Betesh Apr 05 '13 at 18:55
@IsaacBetesh, yep get it `((Date.new(2013+1, 1, 1) - day_offset) << (12-month)).day` works for me :-) – JWL Apr 08 '13 at 06:24
3

`<< (12-month)).day` doesn't seem very readable / understandable / maintainable, whereas `year, month, -1` seems much more obvious - to me. – Michael Durrant Oct 18 '13 at 13:25

score 13 · Answer 5 · answered Mar 04 '15 at 13:58

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

score 9 · Answer 6 · answered Feb 21 '19 at 22:16

9

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

Date.today.all_month.count

answered Feb 21 '19 at 22:16

pierrea

1,368
14
18

1

Docs for `all_month` can be found [here](https://api.rubyonrails.org/classes/DateAndTime/Calculations.html#method-i-all_month). – Marian13 Oct 19 '20 at 01:28

score 7 · Answer 7 · edited Sep 02 '12 at 15:07

7

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

edited Sep 02 '12 at 15:07

Andrew Marshall

95,083
20
220
214

answered Sep 02 '12 at 15:04

Henley

21,258
32
119
207

score 4 · Answer 8 · edited Apr 11 '13 at 10:00

4

revise the input for other format

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

edited Apr 11 '13 at 10:00

CloudyMarble

36,908
70
97
130

answered Apr 11 '13 at 09:43

Mr Bohr

125
3
8

score 3 · Answer 9 · answered Jun 04 '13 at 10:22

3

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)

answered Jun 04 '13 at 10:22

aabes

208
1
12

score 3 · Answer 10 · answered Feb 11 '15 at 21:19

3

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

answered Feb 11 '15 at 21:19

Piotr Karbownik

201
1
4

You could also do `Date.today.end_of_month.day` – 8bithero Apr 18 '16 at 15:12

Alex · Answer 11 · 2017-04-06T15:34:24.820

3

A simple way using Date:

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

edited Apr 06 '17 at 15:34

answered Apr 06 '17 at 13:58

Alex

503
4
7

score 2 · Answer 12 · answered Mar 22 '14 at 08:53

2

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

answered Mar 22 '14 at 08:53

alek

331
1
5

score 1 · Answer 13 · answered Jan 02 '19 at 17:12

1

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

[*Date.today.all_month].size

answered Jan 02 '19 at 17:12

Andres Leon

131
1
8

Time is not irrelevant, the OP wants to know the number of days *in a given month, such as february 1997* – Nino Filiu Jan 02 '19 at 17:35
What I meant is that since we don't need either hours, minutes or seconds then a Time object is not necessary. Therefore, a Date object is more than enough in this case. – Andres Leon Jan 03 '19 at 10:33
I prefer to use a Time object when I can, so as not to have to pull the Date object from the library. – BobRodes Mar 07 '19 at 20:47

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

13 Answers13

Linked