I like that idea. Someone should make a United Kingdom family tree showing how all former territories are the children and grandchildren (and possibly great-grandchildren) of the UK.
Thankfully things have changed since 1914. New Zealand played a pretty shameful part in Samoa's Spanish Flu pandemic. Like other unfortunate parts of New Zealand history, we kind of sweep it under the rug.
It seems that "Date Line" or "Dateline" don't work, only "International Date Line". There is only one Date Line, International or otherwise, so Date Line should be an acceptable alternative.