1 1 [Error] 'day_num' does not name a type

This error occurs when the compiler cannot recognize the identifier `day_num` as a data type.

For example, if you try to declare a variable with the name `day_num` without specifying its data type, the compiler will not know what `day_num` is supposed to represent.

To fix this error, you need to make sure that `day_num` is declared as a valid data type before using it in your code. This can be done by specifying its data type explicitly, such as `int day_num;`, `float day_num;`, or `char day_num;`, depending on the type of value you want to store in `day_num`.