How to calculate DPO

Heather
OK, so i am new to this app. My husband and I are ttc. We got married on 12-13-14! I have a 16 year old son from a prior marriage and he has a 7 year old son. So now we want a child together! And I just turned 40. So, is DPO calculated form the first day of ovulation, the primal day or last day? Thanks in advance!