Guys - All that website is doing is translating the serial number you provide. I have no way of saying it's doing anything illicit but I'd echo others and say stop giving your serial number out to a random website!
Your serial number is formatted like this AABCCDDDEEF, 'BCC' are the numbers you are all wanting, for example '835', literally meaning '2018, week 35'.
So you can just read your serial number and post the week - you do not need that website.
Here is a handy chart that I've copied from one of the other Macrumors threads (iPhone Xs and iPhone Xs Max Antenna Issues Build Date, posted by rebretz000). I used this to determine the build dates for my own and my wife's new iPhone Xs Max phones.