Why everything counts from year 1970?
Not everything counts from 1970.  Its the standard time of Unix.  


  Unix time, or POSIX time, is a system for describing points in time,
  defined as the number of seconds elapsed since midnight proleptic
  Coordinated Universal Time (UTC) of January 1, 1970, not counting leap
  seconds.


Unix Time


  Early versions of unix measured system time in 1/60 s intervals. This
  meant that a 32-bit unsigned integer could only represent a span of
  time less than 829 days. For this reason, the time represented by the
  number 0 (called the epoch) had to be set in the very recent past. As
  this was in the early 1970s, the epoch was set to 1971-1-1.
  
  Later, the system time was changed to increment every second, which
  increased the span of time that could be represented by a 32-bit
  unsigned integer to around 136 years. As it was no longer so important
  to squeeze every second out of the counter, the epoch was rounded down
  to the nearest decade, thus becoming 1970-1-1. One must assume that
  this was considered a bit neater than 1971-1-1.
  
  Note that a 32-bit signed integer using 1970-1-1 as its epoch can
  represent dates up to 2038-1-19, on which date it will wrap around to
  1901-12-13.


Why is 1/1/1970 the “epoch time”?