G'day devbond,

"i need to find the first day of the first week of a month."

Firstly, that seems an odd way to ask this. I'm assuming, for a given year and month, you want to know what day (e.g. Sunday) the 1st of that month is. If that's not what you meant, please clarify.

There's a plethora of modules on CPAN that can do this type of thing; however, the builtin module, Time::Piece, can also do this and you already have it installed. So, unless you're already using some Date::*, Time::*, etc. module, Time::Piece is probably your best choice. Here's sample code:

#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::Piece;

# In 2 days it will be Tuesday, 1st October 2013
my $month = 10;
my $year = 2013;
my $day_name = Time::Piece->strptime("$year$month" . '01', '%Y%m%d')->
+fullday;

print "01-Oct-2013 is a $day_name";
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Output:

01-Oct-2013 is a Tuesday
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It sure is an odd question. I thought he meant "What is the date of the first Sunday (or perhaps Monday) of the month?"

Bill

You could be right. We'll have to wait for devbond to tell us. Anyway, that's just as easy to calculate:

#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::Piece;

my $month = 10;
my $year = 2013;

for (1 .. 7) { # Time::Piece->wday returns: Sunday = 1, ..., Saturday 
+= 7
    print +($_ - Time::Piece->strptime("$year$month" . '01', '%Y%m%d')
+->wday) % 7 + 1;
}
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Output:

6
7
1
2
3
4
5
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Check:

$ cal 10 2013
    October 2013
Su Mo Tu We Th Fr Sa
       1  2  3  4  5
 6  7  8  9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31
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-- Ken

Doing day math with %j which is day of year (yday)

#!/usr/bin/perl --
## perltidy -olq  -csc -csci=10 -cscl="sub : BEGIN END" -otr -opr -ce 
+-nibc -i=4 -pt=0   "-nsak=*"
use strict;
use warnings;
use Time::Piece;
my $now = localtime;
my $year = shift || $now->year;

for my $mon ( 1 .. 12 ) {
    my $first = Time::Piece->strptime( qq/$year-$mon/, q/%Y-%m/ );
    my $mon   = $first->mon;
    my $yday  = $first->yday;
    my( $last ) =
      grep { $_->mon == $mon }
      map { Time::Piece->strptime( qq/$year-$_/, q/%Y-%j/ ) }
      map { $yday + $_ } reverse 28 .. 31;

    printf '%4d %3s %02d/%02d-%02d/%02d  ',
      $first->year, $first->month,
      $first->mon,  $first->mday,
      $last->mon,   $last->mday,;
    print "\n" if 0 == $mon % 3;
}
__END__
2013 Jan 01/01-01/31  2013 Feb 02/01-02/28  2013 Mar 03/01-03/31  
2013 Apr 04/01-04/30  2013 May 05/01-05/31  2013 Jun 06/01-06/30  
2013 Jul 07/01-07/31  2013 Aug 08/01-08/31  2013 Sep 09/01-09/30  
2013 Oct 10/01-10/31  2013 Nov 11/01-11/30  2013 Dec 12/01-12/31
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What is the first week of a month?

See DateTime for doing date/time math.

Also, see Time::Local to get from a date to a timestamp, and localtime to get back to a date. It is often easiest to base your math on "days after the first of a month". Maybe your "first week" starts after the first sunday of the month?

Take a look at Date::Calc. If you're interested in the day of the week a given date falls on, you probably want the Day_of_Week function; ie.:

    $dow = Day_of_Week($year,$month,$day);
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The first day of a week will be determined by culture. It's normally Monday, but I understand that some Christians use Sunday. Followers of other superstitions may use other days for all I know.

Deciding what is the first week of a month is not easy. Consider May 2013, for example, which began on a Wednesday. That's the week that began on Monday the 29th of April. Or consider June 2013, which began on a Saturday, that being in the week that began on Mon 27 May. Most people would say that the first week of May began on the 29th of April, but that the first week of June began on Mon 3 Jun.

ISO 8601 is normally the fount of all wisdom, but in this case isn't particularly helpful as it only defines the first week of a *year*, not of a month - the first week of a year begins on the Monday between Dec 29 and Jan 4, which is the same as saying that it is the one that contains fhe first Thursday of the year. You could adopt that rule for months, but should expect people to disagree with you!