Conception date

When you had sex!

ok thank you so if I had sex on 24th and ovulated on 27th then my conception date to base due date around is the 24th? X

No you're due date is based on the first day of your last period

Initially from the first day of your last period then when you have your scan, this will give you an accurate measurement.

Conception date - date of unprotected sex if after ovulation (and within 5-6 days of ovulation). If before ovulation then it’s the ovulation date. However this is usually a guess.
What NHS would count is the gestation age that is measured from your LMP. If you have longer cycles (over 28 days) then even the gestation age from LMP can be wrong and is best assessed with early scans or a dating scan at 12 weeks if you can wait that long :)