2003/12/03 date

© December 2003 Tony Lawrence

Displays or sets the date.

Setting the date is often difficult for new users: the syntax can be strange and the man page can be confusing. Worse, it is different on the various Unix variants. To set August 25th 2003, 9:15 PM, requires:

• Linux: date 082521152003
• BSD (and Mac OS X) date 200308252115

I often install a script to help:

#!/bin/sh
# Setdate for Linux
echo -n "Enter Date or leave blank for `date +%m/%d/%y`: " 
read mydat
echo -n "Enter Time or leave blank for `date +%H:%M`: " 
read mytime
if [ "$mydat" != "" ]
then
mmdd=`echo $mydat | sed 's/\///g;s/..$//'`
yr=`echo $mydat | sed 's/\///g;s/^....//'`
else
mmdd=`date +%m%d`
yr=`date +%y`
fi

if [ "$mytime" != "" ]
then
newtime=`echo $mytime | sed 's/://g'`
else
newtime=`date +%H%M`
fi
date "$mmdd$newtime$yr"
 

As the above script shows, "date" can be very precise in its output.

On systems where you have it, GNU date ("date -d yesterday") or "datemath" is the easiest way to do such things as finding yesterday's or tomorrow's date. Without that, using Perl or other higher level languages is easier than forcing the issue with "date", but it can be done: https://aplawrence.com/Unix/yesterday.html.

With Perl and other languages, dates are held as they actually are on the system itself: as seconds since January, 1970. This makes date math easy: to get to tommorow, add one day's worth of seconds (60 * 60 * 24), and so on. For example:

#!/usr/bin/perl
$t=time;
print "Today is " . localtime($t);
print "\nYesterday was " . localtime($t - (3600 * 24));
print "\nTomorrow will bes " . localtime($t + (3600 * 24));
 

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