just slightly brush up your math skills and try to compute this expression:
3 - 2 * 24
If you got anything else than (-45), you are wrong as the multiplication has precedence over subtraction.
Oracle, like every other language, also uses the operator precedence. It is stated in the chapter with the same name in the SQL Language Reference book, which is available with other Oracle documentation books e.g. online on http://docs.oracle.com/en/database/database.html Please, consult it.
And, for your own comfort and the sanity of those who will have to maintain the code: use parenthesis on the places where ambiguity may occur to make the expression evaluation order clear.
When you subtract two variables of type TIMESTAMP, you get an INTERVAL DAY TO SECOND which includes a number of milliseconds and/or microseconds depending on the platform. If the database is running on Windows, systimestamp will generally have milliseconds. If the database is running on Unix, systimestamp will generally have microseconds.