In Perl how to find the date of the previous Monday for a given date?

Question

I am looking for a Perl script which can give me the last Monday for any specified date.

e.g. For date 2011-06-11, the script should return 2011-06-06

Answer 1

If you are a slave like me and have to use a corporate Perl without libraries and you are not allowed to install them either, old fashioned way:

my $datestring = "";
my $secondsEpoc = time(); #gives the seconds from system epoch
my $secondsWantedDate;
my $seconds2substract;

$datestring = localtime($secondsEpoc);
print "Today's date and time ".$datestring."\n";
my ($second,$minute,$hour,$d,$M,$y,$wd,$yd) = (localtime)[0,1,2,3,4,5,6,7];
#print "hour: ".$hour;
#print "minute: ".$minute;
#print "second: ".$second;
#print "week day: ".$wd; #week day is 1=Monday .. 7=Sunday

$seconds2substract = 24 * 60 * 60;  # seconds in 24 hours
$secondsWantedDate=$secondsEpoc-$seconds2substract;
$datestring = localtime($secondsWantedDate);
print "Yesterday at the same time ".$datestring."\n";


my $days2Substract = $wd-1;
$seconds2substract = ($days2Substract * 24 * 60 * 60);
$secondsWantedDate=$secondsEpoc-$seconds2substract;
$datestring = localtime($secondsWantedDate);
print "Past Monday same time ".$datestring."\n";

$seconds2substract = ($days2Substract * 24 * 60 * 60) + ($hour *60 *60) + ($minute * 60) + $second;
$secondsWantedDate = $secondsEpoc-$seconds2substract;
$datestring = localtime($secondsWantedDate);
print "Past Monday at 00:00:00  ".$datestring."\n";

Hope it helps somebody!

Answer 2

Zeller's congruence will give you the day of the week. From there it should be pretty easy.

Answer 3

You could use Date::Manip, which has a Date_GetPrev function and understands "Monday"

$ perl -MDate::Manip -le 'print UnixDate(Date_GetPrev(shift, "Monday", 0), "%Y-%m-%d")' 2011-06-11
2011-06-06