Rails calculate date range in months

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How do I calculate the difference of two dates in months? Also, incase it makes a difference, I am working with Date objects, not DateTime. Also, some rounding options might be nice so I can control if I want to round up or down on partial months.

Thanks!

---

Subtracting one Date or DateTime from another will yield the number of days as a fraction, but this can be evaluated as a Float or Fixnum as required.

For instance:

(Date.today - Date.today.advance(:months => -3)).to_f
# => 89.0

There were 89.0 days between today and the same calendar date three months ago. If you work this using 30-day months, or 30.4375 as they are on average, you end up with 2.92 months elapsed between then and now, or rounded up to the nearest integer, 3.

If you want to compute the precise number of calendar months, that is trickier, but can be done.

---

Something like this is more readable than figuring out seconds, and will give you the actual calendar difference:

# Calculate differnce between two dates in months
# Produces b - a
def month_difference(a, b)
    difference = 0.0
    if a.year != b.year
        difference += 12 * (b.year - a.year)
    end
    difference + b.month - a.month
end

If you need to work out the difference based on days as well, you can just follow the pattern

---

We needed something along these lines, but inclusive of partial months. So 1/31 to 2/1 would still yield 2 months. Might help!

def self.month_count(range)
  12 * (range.end.year - range.begin.year) + range.end.month - range.begin.month
end

---

This should give an o.k. approximation:

Date1 - Date2 = difference_in_days
(difference_in_days/30).round = difference_in_months

---

This answer is late to the party, builds on previous answers, and could probably be written more concisely, however, it does give the calendar difference between two dates taking days into account.

def difference_in_months(start_date, today)
  date_to_months(today) - date_to_months(start_date) + adjustment_for_days(start_date, today)
end

def date_to_months(date)
  date.year * 12 + date.month
end

def adjustment_for_days(earlier_date, later_date)
  if later_date.day == earlier_date.day
    0
  elsif later_date.day > earlier_date.day
    1
  else
   -1
  end
end

---

There is a rails helper for this functionality:

http://api.rubyonrails.org/classes/ActionView/Helpers/DateHelper.html#method-i-distance_of_time_in_words

---

how about this practice?

current_date = start_date

while current_date < end_date
  # something  
  current_date = current_date.next_month
end

---

I needed the exact number of months (including decimals) between two dates and wrote the following method for it.

def months_difference(period_start, period_end)
  period_end = period_end + 1.day
  months = (period_end.year - period_start.year) * 12 + period_end.month - period_start.month - (period_end.day >= period_start.day ? 0 : 1)
  remains = period_end - (period_start + months.month)

  (months + remains/period_end.end_of_month.day).to_f.round(2)
end

If comparing let's say September 26th to September 26th (same day) I calculate it as 1 day. If you don't need that you can remove the first line in the method: period_end = period_end + 1.day

It passes the following specs:

expect(months_difference(Date.new(2017, 8, 1), Date.new(2017, 8, 31))).to eq 1.0
expect(months_difference(Date.new(2017, 8, 1), Date.new(2017, 8, 30))).to eq 0.97
expect(months_difference(Date.new(2017, 8, 1), Date.new(2017, 10, 31))).to eq 3.0
# Overlapping february (28 days) still counts Feb as a full month
expect(months_difference(Date.new(2017, 1, 1), Date.new(2017, 3, 31))).to eq 3.0
expect(months_difference(Date.new(2017, 2, 10), Date.new(2017, 3, 9))).to eq 1.0
# Leap year
expect(months_difference(Date.new(2016, 2, 1), Date.new(2016, 2, 29))).to eq 1.0

It relies on Rails' ActiveSupport.