F visa multi entry: does re-entering China extend duration of stay

I have a multi-entry F visa that is valid for one year. It says "duration of each stay: 60 after entry". Does that mean that every time I leave China and then come back I get another 60 days, or do I only get 60 days from the date of my first entry? For example: let's say I entered for the first time on May 15; my visa's duration would run out 60 days later on July 15. However, if I first entered on May 15, but then on June 15 hopped across the boarder to HK and came back, would my visa's duration re-up for a NEW 60 days (now expiring on August 15)? Let me be clear: I am only asking about extending the duration of stay, not about the expiry date of the F-Visa itself. Thanks all!