Not all days are of the same length in all time zones, so $wc * 24 * 3600 is wrong. The simplest workaround is to use a time zone that doesn't have daylight savings time to do date calculations. For example, the following doesn't suffer from this bug: 

sub get_date_by_week {
    my $week = shift || 1;
    my $year = shift || (localtime)[5];
    my $pattern = shift || '%d/%m';
    my $t = timegm(0, 0, 0, 1, 0, $year);
    my $w = (gmtime($t))[6];
    $w = ($w - 2) % 7 + 1;
    my $wc = $week * 7 - $w;
    return strftime($pattern, gmtime($t + $wc * 24 * 3600));
}

[download]

This works, because the answer to "the date x days before date y" is the same in all time zones for a given x and y. x and y are obtained using localtime (as in my $year = shift || (localtime)[5];), but the arithmetic is done using timegm+gmtime.

In reply to Re: Howto get the first day of the week by week number by ikegami
in thread Howto get the first day of the week by week number by nite_man

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.