code: def normalize_timestamp(timestamp=None):
      if timestamp is None:
          timestamp = np.datetime64(datetime.now())
      if isinstance(timestamp, datetime):
          timestamp = np.datetime64(timestamp)
      return timestamp
  
file_path: mars/mars/services/mutable/utils.py
imports: from datetime import datetime
  import itertools
  from numbers import Integral
  from typing import Optional
  import uuid
  import numpy
  from ...core import tile
label_desc: Use image node_id 7 for calling a global function with example usage: normalize_timestamp(timestamp) and returns: timestamp