The date command in Linux boxes is one of the most powerful open source utilities. It is not just for setting the clock on your PC or server, or showing you what the current time is, it can do amazingly more. It can virtually answer all of your chronological questions.

The simplest use case of date command is to view current time, possibly in different time formats –

$ date
Sat Dec 17 00:45:35 EST 2016

$ date '+%Y-%m-%d'
2016-12-17

$ date '+%c'
Sat 17 Dec 2016 12:45:51 AM EST

It is useful in converting time to/from epoch as well –

$ date '+%s'
1481953669

$ date --date='@1481953669'
Sat Dec 17 00:47:49 EST 2016

The most user friendly use case of the date command is the ‘-d’ or ‘–date’ options, which accepts free format human readable date string such as “yesterday”, “last week”, “next year”, “3 min ago”, “last friday + 2 hours” etc. Here is an excerpt from the man page of the GNU date command –

DATE STRING
The --date=STRING is a mostly free format human readable date string such as "Sun, 29 Feb 2004 16:21:42 -0800" or "2004-02-29 16:21:42" or even "next Thursday". A date string may con?
tain items indicating calendar date, time of day, time zone, day of week, relative time, relative date, and numbers. An empty string indicates the beginning of the day. The date
string format is more complex than is easily documented here but is fully described in the info documentation.

Let us play with it –

$ date -d '2 hours ago'
Fri Dec 16 22:51:25 EST 2016

$ date -d '2 hours ago' '+%c'
Fri 16 Dec 2016 10:51:30 PM EST

$ env TZ=America/Los_Angeles date -d '2 hours ago' '+%c'
Fri 16 Dec 2016 07:52:33 PM PST

$ date -d 'jan 2 1990'
Tue Jan  2 00:00:00 EST 1990

$ date -d 'yesterday'
Fri Dec 16 00:53:04 EST 2016

$ date -d 'next year + 2 weeks'
Sun Dec 31 00:53:27 EST 2017

To give a practical example, let us use the date command to get, on which day all the birth days of someone fall, given their date of birth. This can be for past birth days as well as the future. For this example, we will do it from date of birth to this date. Let us pick someone who was born on Feb 29, 1988. This is an edge case. The date command should be smart enough to figure out the leap years.

for year in {1988..2016}; do 
  date -d "feb 29 $year" &>/dev/null
  if [ $? -eq 0 ]; then
    echo -n "Year: $year   " ; date -d "feb 29 $year" '+%c'
  fi
done

Year: 1988   Mon 29 Feb 1988 12:00:00 AM EST
Year: 1992   Sat 29 Feb 1992 12:00:00 AM EST
Year: 1996   Thu 29 Feb 1996 12:00:00 AM EST
Year: 2000   Tue 29 Feb 2000 12:00:00 AM EST
Year: 2004   Sun 29 Feb 2004 12:00:00 AM EST
Year: 2008   Fri 29 Feb 2008 12:00:00 AM EST
Year: 2012   Wed 29 Feb 2012 12:00:00 AM EST
Year: 2016   Mon 29 Feb 2016 12:00:00 AM EST

A typical case would be, say for someone born on Jan 8 1990 –

age=0
for year in {1990..2016}; do 
  echo -n "Age: $age  "; date -d "Jan 8 $year" '+%A %d %B %Y'
  age=$((age+1))
done

Age: 0  Monday 08 January 1990
Age: 1  Tuesday 08 January 1991
Age: 2  Wednesday 08 January 1992
Age: 3  Friday 08 January 1993
Age: 4  Saturday 08 January 1994
Age: 5  Sunday 08 January 1995
Age: 6  Monday 08 January 1996
Age: 7  Wednesday 08 January 1997
Age: 8  Thursday 08 January 1998
Age: 9  Friday 08 January 1999
Age: 10  Saturday 08 January 2000
Age: 11  Monday 08 January 2001
Age: 12  Tuesday 08 January 2002
Age: 13  Wednesday 08 January 2003
Age: 14  Thursday 08 January 2004
Age: 15  Saturday 08 January 2005
Age: 16  Sunday 08 January 2006
Age: 17  Monday 08 January 2007
Age: 18  Tuesday 08 January 2008
Age: 19  Thursday 08 January 2009
Age: 20  Friday 08 January 2010
Age: 21  Saturday 08 January 2011
Age: 22  Sunday 08 January 2012
Age: 23  Tuesday 08 January 2013
Age: 24  Wednesday 08 January 2014
Age: 25  Thursday 08 January 2015
Age: 26  Friday 08 January 2016
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