Counting the number of X in a given period of Time

[Matthew Ervin]

CONTEXT: I'm setting up a graduate student records database and I'm adding a component that tracks interactions between the program coordinator and students, e.g., "I had a conversation with X about Y" the student is linked to a description of the conversation (and it is capable of linking multiple students for when they come in groups bearing torches).

PROBLEM: I'd like to add fields which count the number of interactions a particular student has had in the last six months, semester, and year. I can't for the life of me figure out how do implement this.