Useful Date Algorithms

[Sol-Invictus]

Description:

Date calculations : yes there are modules for this on CPAN, but wouldn't you like to know how to do this kind of stuff :)

Some algorithms useful for calculating:

• leap years
  
• the day of the week a given dates fell on
  (but bear in mind the exact dates when countries started changing from Julian to Gregorian vary, more info here).

#!/usr/bin/perl -w

#=========== declare includes =============

use strict;
use diagnostics-verbose;



#========== declare variables =============

my($Adjuster,$GregorianDay,$JulianDay,$LastOfMonth,$day,$month,$year);
my(@DaysOfWeek);




# Need to print out  calendar for a given year?
# 30 days hath September, April, June and November
# all the rest have 31
# except February 28 days clear, 29 each leap year
# the rule for working out leap years =
# If the year is divisible by 100 it's NOT a leap year
# If the year is divisible by 4 or 400 it's a leap year

$LastOfMonth = '30'if ($month == (4||6||9||11));
$LastOfMonth = '31'if ($month == (1||4||5||7||8||10||12) );

if ($month == 2){
                
 if ( $year % 100 != 0 && $year % 4 == 0 || $year % 400 == 0 ){
    $LastOfMonth = '29';
    } else {  $LastOfMonth = '28'; }
}




# The programmatic way to convert the date into the name for the day o
+f the date given
# this script expects to be fed the date, which it will then output a 
+number 
# which represents Julian day and Gregorian day with Sunday=0
# Julian days are useful for calculating dates before the change to th
+e Gregorian Calendar


$day= 1;
$month=3;
$year=2003;


$Adjuster = int    (     (    14-$month    )/12     );
$month = int ( $month +(    (    $Adjuster*12    )-2    )     );
$year =  int ( ( $year -$Adjuster )    );

$JulianDay = (5+$day+$year+int($year/4)+int(31*$month/12) ) %7;
$GregorianDay = ($day + $year + int($year/4) - int($year/100) + int($y
+ear/400) +int (31*$month/12) ) %7;



print "The value of \$JulianDay is: ",$JulianDay,"\n";
print "The value of \$GregorianDay is: ",$GregorianDay,"\n";


# use the value output on an array like this to get the day of the wee
+k

@DaysOfWeek=qw(    Sunday
                Monday
                Tuesday
                Wednesday
                Thursday
                Friday
                Saturday
                );