Q: How long does a calendar day exist on Earth?
A: 48 hours
24 hours is the expected answer but it is incorrect.
Today, for example, is February 3rd, 2015. As midnight Feb 2nd fell upon the International Date Line, Feb 3rd came into existence. For the next 24 hours, Tuesday the 3rd ‘grew’ out from the 180° longitude and will eventually encircle the globe. For one infinitely short instant every place on Earth will share the same date. At that point this calendar day will have existed for a full 24 hours. It will then take another full 24 hours for Feb 4th to encircle the globe, taking that time to completely ‘replace’ Feb 3rd..
So if you wanted a particular day (birthday, honeymoon, anniversary ,etc…) to last as long as possible, just fly west from the International Date Line half as fast as the world turns. You could then be in the same calendar date for a full 48 hours.