Ruby code to get the date of next Monday (or any day of the week)

Given an input of, for example,

day = 'Monday'

how can I calculate the date of the next day?

def date_of_next(day)
  ...
end

require 'date'

def date_of_next(day)
  date  = Date.parse(day)
  delta = date > Date.today ? 0 : 7
  date + delta
end

Date.today
#=>#<Date: 2011-10-28 (4911725/2,0,2299161)>
date_of_next "Monday"
#=>#<Date: 2011-10-31 (4911731/2,0,2299161)>
date_of_next "Sunday"
#=>#<Date: 2011-10-30 (4911729/2,0,2299161)>

For anyone like me who came here looking for a solution in Rails to this problem, as of Rails 5.2 there is a much easier method to do this.

For anyone (like the original poster) not specifically using Rails, this functionality is available in the ActiveSupport gem.

To find the next occurring day of a week, we can simply write something like Date.today.next_occurring(:friday).

See the documentation for more details.

I know this is an old post, but I came up with a couple of methods to quickly get the previous and next day of the week.

# date is a Date object and day_of_week is 0 to 6 for Sunday to Saturday

require 'Date'

def get_next_day(date, day_of_week)
  date + ((day_of_week - date.wday) % 7)
end

def get_previous_day(date, day_of_week)
  date - ((date.wday - day_of_week) % 7)
end

puts today = Date.today
# 2015-02-24

puts next_friday = get_next_day(today, 5)
# 2015-02-27

puts last_friday = get_previous_day(today, 5)
# 2015-02-20

Rails >= 5.2.3

Date.current.next_occurring(:monday)

https://api.rubyonrails.org/classes/DateAndTime/Calculations.html#method-i-next_occurring

For stuff like this I rely on the chronic library.

The Ruby code would be:

def date_of_next(day)
    Chronic.parse("next #{day}")
end

If you are using rails you can use Date.today.sunday for Sunday or Date.today.monday for Monday. And then Date.today.sunday - 1.day for Saturday etc.

A Rails 4-compatible solution:

(Date.today + 1.week).beginning_of_week(:monday)

where you can provide the day you'd like to find as a symbol argument; the default is :monday.

Note that this will find the next occurrence of the given day - if today is Monday and you're looking for the next Monday, it will return the Monday one week from today.

Some equivalent ways to do it:

1.week.from_now.beginning_of_week(:monday).to_date

1.week.since(Date.today).beginning_of_week(:monday)

(source: https://apidock.com/rails/Date/beginning_of_week/class)