Unix time calculates the number of seconds elapsed since that date. But here comes the problem. The standard Unix time_t (data type representing a point in time) is a signed integer data type, traditionally of 32 bits. This means maximum amount of seconds that unix time based devices can calculate will eventually run out on 19 January 2038 ! Think of it as the next Y2K bug, some even call it the UNIX millennium bug. Its such a big problem that xkcd even made a comic on it :
|Courtesy : xkcd|
Fortunately modern 64 bit system use signed 64-bit time_t integers which enable them to display time approximately 292 billion years from now. So you won't have to worry till they year December 292,277,026,596. Check out more information on the Year 2038 problem here.