def humanize_seconds(seconds):
  """
  Returns a humanized string representing time difference
  between now() and the input timestamp.
  
  The output rounds up to days, hours, minutes, or seconds.
  4 days 5 hours returns '4 days'
  0 days 4 hours 3 minutes returns '4 hours', etc...
  """  
  minutes, seconds = divmod(seconds, 60)
  hours, minutes = divmod(minutes, 60)
  
  if hours > 0:
    if hours == 1:  return "{0} hour".format(hours)
    else:           return "{0} hours".format(hours)
  elif minutes > 0:
    if minutes == 1:return "{0} minute".format(minutes)
    else:           return "{0} minutes".format(minutes)
  elif seconds > 0:
    if seconds == 1:return "{0} second".format(seconds)
    else:           return "{0} seconds".format(seconds)
  else:
      return None