Per guidance from our International Board Certified Lactation Consultant (IBCLC), it is universally accepted and advised to count the length between feedings from the start of one feeding to the start of the next. For example, when your pediatrician or lactation consultant recommends feeding your baby every 2 hours they literally mean - if your baby's feeding began at 12:00pm the next feeding should begin at 2:00pm. This standard interpretation of feeding intervals helps ensure parents and providers are "speaking the same language" when discussing feeding.
Still you may wonder, "why do experts agree we do it like this?" The determination was made as a way to account for varying lengths of the actual feeding. Depending on the age of your baby, what their eating (i.e. breast milk or formula), and the method of feeding (i.e. breast or bottle), feeding lengths can vary greatly. In the early weeks many newborns will feed for 30 minutes or longer. Older babies may only feed for 10 minutes or less.
Another way to think about this is to relate it to adult lifestyles and how you describe your own meal times. If you were meeting up with a friend for dinner and discussing how hungry you are you might say to your friend, "I'm very hungry, I haven't eaten since 11:00am." Of course you and your friend both accept thatyou mean 11:00am is when you began lunch. You may not have finished your last bite until 11:30am.
Hence when you see on the Home Screen "2h 9 m ago" forexample (see screenshot), this represents counting the last feeding from the start time.