I don't know who thought up the term 'Down Under' but it refers to a geographical position on the earth. According, to the Oxford dictionary* 'down under' is a colloquial term which means "in or to the antipodes, i.e. to Australia or New Zealand" (p.296). Antipodes comes from a Greek word meaning "having the feet opposite"; and in modern times meaning "a place diametrically opposite to another, esp. Australia relative to Europe" (Oxford, p.39). So, Australia and other countries down under in the Southern hemisphere are regarded as the 'feet end' of the earth 🙃. This is a bit geo-centric off course, as there is really no up or down on earth or in space that we know of. Now, having set the scene, picture yourself looking at a world globe at the down under region. Now, imagine that you are in space looking at the real earth from this perspective. Imagine moving further and further away from the earth until you have reached the Southern Cross constellation. Looking down at the earth now from the Southern Cross, with your special telescope, all you see is a huge sun shining down on a tiny rotating earth. You cannot see the whole earth, only the Southern down under side facing you, just as only people who live down under can see the Southern Cross. In this way, the Down Under Earth Time Clock only shows one side of the earth (the south), yet includes all countries with respect to their position of longitude.
Still don't understand? Let me explain in another way. One evening as I was sitting on my front porch looking at the Southern Cross constellation, I imagined I was up there looking at the earth while it rotated under the sun. It rotated clockwise and formed a giant earth time clock. Actually, there is a bit more to the story but I'll get to that another time ...
Cheers Vicky (down under 🙃)