It should be 19, because "today's math exam", then "today" will not end after the exam, and in terms of time, the exam will definitely end in the afternoon, not at midnight 12, so the exam will end first, and then today.

Moreover, it doesn't mean that "exam" refers to one day, "the next day" refers to another day, and "today and the day after tomorrow" refers to "the day after the math exam", so the number 18 "exam" and the number 19 are the day after the exam.

On the other hand, it is also understood from Chinese language habits and usage habits ~

I hope it helps you ~