I'm not totally sure, but I believe there is a date/time function that will return the difference between two times. I'm not at my workstation, so can't test/double check syntax but something along the lines of this should be a good start.
Code:
SELECT DATETIMEDIFF( (SELECT time FROM crimes WHERE ....), NOW());
Assuming you store each crime (and date) in the database. I'm probably wrong on the function name, but check the date and time functions in the docs for better info.