Is it time for a new toothbrush?

Dentists (and toothbrush manufacturers) recommend a new toothbrush every three months. My problem was remembering when three months is up. Since I always have at least one extra toothbrush on hand, my solution is to write on the next brush's box the date I opened the previous one on. So on the spare brush I write June 14, and the next time I'm wondering when my three months is up, I can just check the box and see that I don't need to switch until September 14.