===== Epoch time calculations =====

==== Days since Epoch ====

  echo $(date +%s)/86400 | bc
  echo $(($(date +%s)/86400))
  let days=$(date +%s)/86400 ; echo $days
 
==== Days since Epoch without GNU date ====

If perl is installed:

  perl -e ' $days=int(time()/86400);print "$days\n"; '


==== Current Epoch time on system without GNU date ====

  perl -e 'print time(), "\n" '
  
==== Convert from Epoch to date with GNU date  ====

  date -d @<EPOCH time>
  Example:
  date -d @1368023418
  Wed May  8 16:30:18 CEST 2013


==== Convert from Epoch to date with perl ====
  
  perl -e "print scalar(localtime(1268727836))"
  
==== Convert date to epoch ====

  date -d 'Apr 30 14:40:09 2012' +%s

==== Convert date 14 days ago to epoch ====

   date -d '14 days ago' +%s

==== Find date for last password change for Linux user ====

Replace <username> with account in question

  passwd -S <username>
  <username> PS 2013-02-27 7 90 7 14 (Password set, SHA512 crypt.)

  date -d "1 January 1970 + $(grep <username> /etc/shadow | awk -F: '{print $3}') days"

===== Convert UTC date/time to local timezone  =====
  
  date -d '2012-08-08 16:00 UTC'
  Wed Aug  8 18:00:00 CEST 2012
  
  date -d '2012-12-08 16:00 UTC'
  Sat Dec  8 17:00:00 CET 2012
  
===== Convert current local time to UTC  =====
  
  date --utc
  Wed Aug  8 12:35:15 UTC 2012