fields is an tuple containing instances of the class DateField, created with 3 arguments: input_formats set to input_date_formats,
fields = ( DateField ( input_formats = input_date_formats , error_messages = { 'invalid' : errors [ 'invalid_date' ] } , localize = localize ) , TimeField ( input_formats = input_time_formats , error_messages = { 'invalid' : errors [ 'invalid_time' ] } , localize = localize ) , )