Curval, I just discussed this matter with my wife, telling about the website in question. For her it was clear as daylight: Simply calculate 280 days backwards from your birthday, add some days before and after. The same as the first link does. They teach that on eighth grade or so (no need to discuss it with your parents) and should I ask my daughters (16 and 19) about it, they'd look at me with a pitiful look and say that it's common knowledge and as a father I should know it.
I also noticed that you've been here for fourteen years, which I believe means you should be way over 20 now. I suppose by now you already know how children are made instead of believing in the Delivery Stork.