Are we talking about Excel here? If that is the case Excel does an automatic detection of the information and gives it a best effort to format it. In your case that would be the American date format.
If you want Excel to calculate you need to put in =12-24. If you want it to be a string there are two ways. One is to put quotes around your numbers "12-24" and the the second is to use the ribbon to format the cell as text before adding anything into it.