If I am 30 weeks when did I conceive my child

Around Dec 17th.

Yes, but remember when the doctor says 30 weeks that is a quess depending upon info that you gave them and at times not exact. So often give or take a week or even two sometimes. So if the issue is two possible fathers and the time frame is within a week or two, to be honest you can not be 100 percent sure till you have a DNA test.
*** I know you did not ask them but this is norally why people ask this question