1 xz is a command line tool with syntax similar to that of gzip. 2  xzdec is a decompression-only tool smaller than the full-featured xz tool. 3  A set of shell scripts (xzgrep, xzdiff, etc.) have been adapted from gzip to ease viewing, grepping, and comparing compressed files.
Epoch (astronomy), a moment in time used as a reference for the orbital elements of a celestial body. Epoch (cosmology) or cosmologic epoch, a phase in the development of the universe since the Big Bang. Epoch (geology) or geologic epoch, a span of time smaller than a period and larger than an age.
The Epoch is January 1st, 1970, midnight, on UNIX systems. On other systems, look at the results of time.gmtime(0) to discover the date of the Epoch. Measuring time in seconds since the Epoch is convenient for storage and comparing dates, because you only have a single number to consider. The tuple contains several values, arranged the following way: year, month 1-12, day 1-31, hour 0-23, minutes 0-59, seconds 0-61, day 0-6 (Mon-Sun), day 1-366, and DST -1,0,+1.