code: def test_combine_first_timestamp_bug(scalar1, scalar2, nulls_fixture):
      # GH28481
      na_value = nulls_fixture
  
      frame = DataFrame([[na_value, na_value]], columns=["a", "b"])
      other = DataFrame([[scalar1, scalar2]], columns=["b", "c"])
  
      common_dtype = find_common_type(
          [frame.dtypes["b"], other.dtypes["b"]]
      )
  
      if (
          is_dtype_equal(common_dtype, "object")
          or frame.dtypes["b"] == other.dtypes["b"]
      ):
          val = scalar1
      else:
          val = na_value
  
      result = frame.combine_first(other)
  
      expected = DataFrame(
          [[na_value, val, scalar2]], columns=["a", "b", "c"]
      )
  
      expected["b"] = expected["b"].astype(common_dtype)
  
      tm.assert_frame_equal(result, expected)
  
imports: from datetime import datetime
  import numpy
  import pytest
  from pandas.core.dtypes.cast import find_common_type
  from pandas.core.dtypes.common import is_dtype_equal
  import pandas
  from pandas import DataFrame, Index, MultiIndex, Series
  import pandas._testing
label_desc: Use image node_id 1 for calling a global function with example usage: test_combine_first_timestamp_bug(scalar1, scalar2, nulls_fixture) without return types