Sign out from your Yahoo Messenger or Google Talk. Change your Calendar year beyond 2038 say 2040 in your Windows O.S. then log in once again to these messengers what happens? . Messengers would fail! Unix    Millennium  2038  Bug  is  a  problem which may seems to be a small   but  can  cause a problem to wide range of devices and Machines.           Unix  was  built  in   1969    specifically    on    20th April 1969. During    that   period   Unix  developer  didn’t know that Software    can  live    for    such   a long time (greater than 100 years).But today we have some   geeks  software  available  for e.g. Microsoft office Notepad  which simply seems to live even greater than hundreds of year from now on.           So  problem    is  that   during    1970   UNIX   developer developed time_t. This Object is of 32bit.   So it can store 2^32 values of int type, basically which is  equivalent  to  137  years. But since time_t is unsigned   it   can    store  +68years  (and 19 days), -69years.  So  range  of  time_t is 1901 to 19th Jan. 2038.  After 2038 time_t would  simply   rollback to 1901 Hence forth all software dependent on time would fail to operate correctly.