You may know that the Unix timestamp was limited to the years 1970 to 2038. You may ask why you are limited to the years 1970 through 2038. Well, it’s because the original developers of Unix chose the start of the year 1970 as the base date that no programmer should need to go before! The Unix designers also decided that nobody would be using Unix after about 70 years or so, and therefore believed they could get away with storing the timestamp as a 32-bit value—which will run out on January 19, 2038! This will create what has come to be known as the

Y2K38 bug (much like the millennium bug, which was caused by storing years as two-digit values, and which also had to be fixed). We have to hope it will all be solved well before we get too close to that date.

http://en.wikipedia.org/wiki/Year_2038_problem