She did not have period this month... I tested with prega news.. it was positive... but how to know the exact time of getting pregnant is there any test to find out the exact time of pregnancy?

No, it was sometime after the last period, to the time she missed this period.

Even after the fetus is larger and they do estimated delivery dates, they give a estimated conception dates, but it is based on info you provide, and that date can be give or take two weeks.

Normally actual conception dates is not a issue, since she will know the dates she had sex