code: def start_time_str(start_time):
      """Get string representation of the start time."""
      return start_time.strftime("%Y-%m-%dT%H:%M:%SZ")
  
imports: import datetime
  import os
  import numpy
  import pytest
  import xarray
  from pyresample.geometry import AreaDefinition
  from satpy.tests.utils import make_dataid
label_desc: Use image node_id 2 for calling a global function with example usage: start_time_str(start_time) and returns: start_time