2 entries for 30 days

i just want to clear something because i have 2 entries i entered china in feb.28 and exit when i go in hk in march 14 (15days stay) then re-entered again china on same date March 14. is my visa expire on march 29 coz i have only 30 days to stay in china. or i have another 30 days to stay in china.i'm just confused! thanks