Happy New Year and Atomic Clocks?

I woke this morning thinking about New Year and how it happens at different times around the globe from where you actually live. 'What?'  Let me explain ...  the fist place on earth to enter the New Year are the Christmas Islands, the last place to enter New Year is Samoa. Britain enters the New Year about 13 hours between them. Its odd don't you think that Earth celebrates a new New Year for a full 24 hours. Well its about 24 hours but not exactly 24 hours because we have been loosing time at the rate of a couple of thousandths of a second every century for quite some while now - so its not really 24 hours at all. But how do we really know what time the correct time is?  We use 'TIME' as a constant to measure so many other things and we rely on it being right... right? So how do we measure time?   Since the invention of the atomic clock in 1955 we can now rest assured that based on the properties of atoms we know  'time' is being measured correctly. I'm not sure how we did it before and when we switched from what we did then to the atomic clock how do we know that the time we switched on the the atomic clock was right in the first place?   Well I've no idea  .. all I know now is its time to celebrate New Year in Catherington in about 15 minutes ... Happy New Year!