I'm trying to understand why Macaulay Duration is recalculated after each coupon payment. I can see why I want to know the duration when purchasing a bond - to figure out how long till I recoup my initial investment. Yet the formula continuously recalculates and grows as each coupon is received.
Should I be keeping the Macaulay Duration result and keeping a running total to know when it has elapsed and using the latest Macaulay Duration to keep the Modified Duration up to date? Or am I misusing this formula entirely?