time_helpers.rb

def interpret_time(time)
  return nil if time.blank?
  return time if time.kind_of?(Time) or time.kind_of?(Date)
  named_time(time) || time_by_regex(time) || Time.parse(time)
end

def named_time(time)
  case time.downcase
  when "today" : Date.today
  when "now" : Time.now
  when "tomorrow" : Date.tomorrow
  when "yesterday" : Date.yesterday
  else nil
  end
end

def time_by_regex(time)
  directional_regex = /(ago|from now)$/
  time_regex = /(\d+) (minute|hour|day|week|month|year)s?/i

  direction= time.scan(directional_regex).flatten
  shifts = time.scan(time_regex)

  return nil if direction.empty? or shifts.empty?

  result = Time.now
  backward = (direction.first == "ago")
  shifts.each do |count, unit|
    adjustment = count.to_i.send(unit)
    result = backward ? (result - adjustment) : (result + adjustment)
  end

  result
end

usage.feature

  Given a document posted today
  Given a document posted 2 months, 5 days ago
  Given my last login was 3 hours ago
  Given a task due 1 week from now