It begins the first day of active duty. Reserve time also contributes toward retirement, but at a much slower (almost nonexistant) pace. Just out of curiosity….why would you enlist if you’ve got a bachelor’s already? Most people go officer, because your pay is MUCH higher. If you’re already thinking pension time, think about this: even the lowest ranking officer makes about as much in basic pay as many enlisted who have been in 10 or 15 years. Your retirement pay is based on the highest 3 years of pay. Are you sure you want to put that limit on your retirement pay right from the word go? I haven’t met many people who didn’t kick themselves later for enlisting when they should have been commissioned. Not telling you what to do, just saying. Also, they don’t typically care what your degree is in once you’re in. It’s cool that you have one, but you should really look into other options (not necessarily avoiding military–but maximize what the military will give you).