My solution:

key-value-store.rb

require 'date'

class KeyValueStore
 attr_accessor :record

  def initialize()
    @record = {
      'Bob' => {
        (DateTime.now - (100/1444.0)) => 'A',
        (DateTime.now - (50/1444.0)) => 'B'
      },
      'Sharon' => {
        (DateTime.now - (50/1444.0)) => 'V',
        (DateTime.now - (100/1444.0)) => 'O',
        (DateTime.now - (19/1444.0)) => 'P',
        (DateTime.now - (10/1444.0)) => 'K',
        (DateTime.now - (100/1444.0)) => 'I'
      }
    }
    # I initialized some sample data
  end

  def put(k,v)
    if !@record[k]
      @record[k] = {}
      @record[k][DateTime.now] = v
    else
      @record[k][DateTime.now] = v
    end
  end

  def get(k,timestamp)
    # @record[k].keys == @record[k].keys.sort # this suggests that the keys are already sorted in the hash
    # return @record[k].values[-1] if timestamp == nil # since the timestamp keys have already been sorted, I just pulled all the values (which comes back as an array), and returned the last value because we want the latest value if no timestamp is given
    # but if we were to add key/value pairs and have the datetime not DateTime.now, I would have to give the timestamp a value for comparsion
    if timestamp == nil
      timestamp = DateTime.now
    end
   
    record_keys = @record[k].keys.sort!
    result = ''

    record_keys.each_with_index do |key, index|
      if record_keys[index] < timestamp && record_keys[index+1] == nil || record_keys[index] < timestamp && timestamp < record_keys[index+1]
        result = record_keys[index]
      end
    end
    @record[k][result] ? @record[k][result] : nil
  end

  def peek
    @record
  end
end

store = KeyValueStore.new
store.put('Bob', 'C')
store.put('Bob', 'D')
store.peek
store.get('Bob', nil)
p store.get('Sharon', nil)
store.get('Sharon', DateTime.now)
store.get('Sharon', (DateTime.now - 15/1444.0))
store.peek
# to see the output of the test code, just put the letter 'p' in front of it
# for example: 'p store.peek'