Freeze Time test cases fail when a variable is used to reference the time

I'm using freeze-time to run my python unittest Test cases.

A dummy test case:

@freeze_time('2020-01-01')
def test_something(self):
  expected_output = {'time': '2020-01-01'}

  output = call_tested_code()

  self.assertEqual(expected_output, output)

Main code / Code that is being tested:

GET_CURRENT_TIME = datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S')

def call_tested_code():
  return {'time': GET_CURRENT_TIME}

This is failing, as the output is giving current_date instead of frozen date. It was working when the GET_CURRENT_TIME was a lambda, but that results in different timestamps for my code, which I don't want.

Feel free to comment if any additional Information is required. Thanks