A Ruby function for determining the number of whole years elapsed between two `Time` objects. No dependencies. MIT License.

years-elapsed.rb

##
# Determine the number of whole years elapsed between two [Time] objects.
# It is assumed that the provided [Time] objects are in the same time-zone
# and part of a single continuous chronology.
#
# @param time1 [Time]
# @param time2 [Time]
#
# @return [Integer]
#
# Copyright 2018 Ry Biesemeyer (@yaauie)
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.
#
def years_elapsed(time1, time2)
  fail(ArgumentError, "Invalid `time1` value `#{time1.inspect}` (expected a `Time` object)") unless time1.kind_of?(Time)
  fail(ArgumentError, "Invalid `time2` value `#{time1.inspect}` (expected a `Time` object)") unless time2.kind_of?(Time)

  # normalize order
  time1, time2 = time2, time1 if time1 > time2

  years = time2.year - time1.year

  [:month,:day,:hour,:min,:sec,:subsec].each do |precision|
    case (time2.send(precision) - time1.send(precision))
    when 1  then return years
    when -1 then return years -1
    end
  end

  return years
end