I don't trust your link in the slightest.  It's not even from a source at the UK Government, or from a reliable commercial source, like a Zimbabwean vendor.  The longest day can vary based on your latitude, occasionally longitude, and your altitude relative to cloud cover levels over certain mountain peaks and the presence of reptiles and other large fauna.  Things are not as simple as you make them out to be, my friend (hope you don't mind me calling you that), but the fact remains, facts are fact, and the change regarding May 23rd was identified as one by someone at the Wikipedia machine.  With editors such as you at the helm, I trust that implicitly.  I also trust the authorized and authenticated go code I had to make such a posting.  The facts are indisputable.  Best wishes.