input: 
  from flask import Flask
  import datetime
  
  def create_datetime_api():
      """
      Create a web API using Flask that provides the current date and time.
      The API should have a single endpoint '/datetime' which returns the
      current date and time in the format "YYYY-MM-DD HH:MM:SS".
  
      Example:
      If the current date and time is January 1, 2023, 12:34:56 PM, the endpoint
      should return '2023-01-01 12:34:56'.
      """
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
      if __name__ == '__main__':
          app.run()
  
  # Example output:
  # If called at January 1, 2023, 12:34:56 PM, the API would return '2023-01-01 12:34:56'.
  
output:     return app