The logical study and application of basic mathematical principles to deduce when your significant other was last with their ex-boyfriend or girlfriend.
While founded in the scientific theory, ex-math should not be considered for a major field of study as the process is often characterized by the absence of objective and rational thought.
Girlfriend: "I wondered after I left about the age you said you were when you were with XX so I put everything in a calendar. You said you were 24 so that was a couple of months before me? :-) sorry. I try not to be curious about stuff as that is typically not a good thing. But I wondered it and so I'm just...um..saying that I was curious."
Boyfriend: "Well if you're going to use ex-math, put it on a Chinese calendar. That way I haven't been with her since last year. Does that make you feel better?"