Calculate number of business days between two days

Take a look at business_time. It can be used for both the things you're asking.

Calculating business days between two dates:

wednesday = Date.parse("October 17, 2018")
monday = Date.parse("October 22, 2018")
wednesday.business_days_until(monday) # => 3

Adding business days to a given date:

4.business_days.from_now
8.business_days.after(some_date)

Historical answer

When this question was originally asked, business_time didn't provide the business_days_until method so the method below was provided to answer the first part of the question.

This could still be useful to someone who didn't need any of the other functionality from business_time and wanted to avoid adding an additional dependency.

def business_days_between(date1, date2)
  business_days = 0
  date = date2
  while date > date1
   business_days = business_days + 1 unless date.saturday? or date.sunday?
   date = date - 1.day
  end
  business_days
end

This can also be fine tuned to handle the cases that Tipx mentions in the way that you would like.

business_time has all the functionallity you want.

From the readme:

#you can also calculate business duration between two dates

friday = Date.parse("December 24, 2010")
monday = Date.parse("December 27, 2010")
friday.business_days_until(monday) #=> 1

Adding business days to a given date:

some_date = Date.parse("August 4th, 1969")
8.business_days.after(some_date) #=> 14 Aug 1969

We used to use the algorithm suggested in the mikej's answer and discovered that calculating 25,000 ranges of several years each takes 340 seconds.

Here's another algorithm with asymptotic complexity O(1). It does the same calculations in 0.41 seconds.

# Calculates the number of business days in range (start_date, end_date]
#
# @param start_date [Date]
# @param end_date [Date]
#
# @return [Fixnum]
def business_days_between(start_date, end_date)
  days_between = (end_date - start_date).to_i
  return 0 unless days_between > 0

  # Assuming we need to calculate days from 9th to 25th, 10-23 are covered
  # by whole weeks, and 24-25 are extra days.
  #
  # Su Mo Tu We Th Fr Sa    # Su Mo Tu We Th Fr Sa
  #        1  2  3  4  5    #        1  2  3  4  5
  #  6  7  8  9 10 11 12    #  6  7  8  9 ww ww ww
  # 13 14 15 16 17 18 19    # ww ww ww ww ww ww ww
  # 20 21 22 23 24 25 26    # ww ww ww ww ed ed 26
  # 27 28 29 30 31          # 27 28 29 30 31
  whole_weeks, extra_days = days_between.divmod(7)

  unless extra_days.zero?
    # Extra days start from the week day next to start_day,
    # and end on end_date's week date. The position of the
    # start date in a week can be either before (the left calendar)
    # or after (the right one) the end date.
    #
    # Su Mo Tu We Th Fr Sa    # Su Mo Tu We Th Fr Sa
    #        1  2  3  4  5    #        1  2  3  4  5
    #  6  7  8  9 10 11 12    #  6  7  8  9 10 11 12
    # ## ## ## ## 17 18 19    # 13 14 15 16 ## ## ##
    # 20 21 22 23 24 25 26    # ## 21 22 23 24 25 26
    # 27 28 29 30 31          # 27 28 29 30 31
    #
    # If some of the extra_days fall on a weekend, they need to be subtracted.
    # In the first case only corner days can be days off,
    # and in the second case there are indeed two such days.
    extra_days -= if start_date.tomorrow.wday <= end_date.wday
                    [start_date.tomorrow.sunday?, end_date.saturday?].count(true)
                  else
                    2
                  end
  end

  (whole_weeks * 5) + extra_days
end