new_contents: import pytest
  import diff_between_times
  
  def test_validate_date():
      val = ('2021-09-15 00:00:00')
      assert diff_between_times.validate_date(None, None, val) == val
      
      with pytest.raises(Exception):
          val = ('2021-09-15 00:')
          assert diff_between_times.validate_date(None, None, val)
      
      since_val = '2021-09-15 00:00:00'
      until_val = '2021-09-15 00:01:00'
      assert diff_between_times.dateDiff(since_val, until_val) == 1
  
subject: Create tests_diff_between_times.py