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Adjusting Date By Number Of Days

By Will Bontrager

Unix system time is the number of seconds elapsed since January 1, 1970.

Dates and times can be calculated from a system time number, with Perl, using the localtime() function. The "Converting and Formatting System Time" blog post has more information.

To adjust a date by a certain number of days, convert the date to a system time number, adjust the system time number by 86400 for each day, then convert the result into a date. (86400 is the number of seconds in a day.)

Here is a Perl script to do that. The script starts with a date. It then prints the date 90 days later and the date 90 days earlier.

#!/usr/bin/perl
use strict;
use Time::Local;

sub CalendarDateToSystemTime
{
   # assumes month numbers are 1=January, 12=December
   my($day,$month,$year) = @_;
   $year -= 1900 if $year > 1900;
   return timelocal(0,0,0,$day,($month-1),$year);
} # sub CalendarDateToSystemTime

sub SystemTimeToCalendarDateAdjusted
{
   my ($systemtime,$daysadjustment) = @_;
   $systemtime += ($daysadjustment * 24 * 60 * 60);
   my ($sec,$min,$hour,$day,$month,$year,$wday,$yday,$isdst) = localtime($systemtime);
   my @monthname = qw(January February March April May June July August September October November December);
   return ($day,$monthname[$month],$year+1900);
} # sub SystemTimeToCalendarDateAdjusted

my($Day,$Month,$Year) = (23,2,2009); # 23rd of February, 2009
my $SystemTime = CalendarDateToSystemTime($Day,$Month,$Year);
print "Content-type: text/html\n\n<html><body><pre>23rd of February, 2009\n";
($Day,$Month,$Year) = SystemTimeToCalendarDateAdjusted($SystemTime,+90);
print "+90 days = $Day-$Month-$Year\n";
($Day,$Month,$Year) = SystemTimeToCalendarDateAdjusted($SystemTime,-90);
print "-90 days = $Day-$Month-$Year\n";
print '</pre></body></html>';

May 1, 2008

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Adjusting Date By Number Of Days

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Adjusting Date By Number Of Days

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