Ovulation calculation

Barbara
So I been trying this ovulation calculation sites like web MD, n first response, n they tell me that my ovulation starts later than glow tells me, so I'm a little confuse as which one to follow glow indicates that I started ovulation 4 days ago, the others say I start ovulation today 2-28-15 I don't want to miss the opportunity even though I'm not feeling any ovulation symptoms.