Howto get the first day of the week by week number
Today I faced with small problem. I prepared a chart where data was split by weeks. It was very easy except one thing: display the first day of the week instead of week number. Here is a one way to do it (the main idea belongs to my co-worker Igor):
sub get_date_by_week {
my $week = shift || 1;
my $year = shift || (localtime)[5];
my $pattern = shift || ‘%d/%m’;
my $t = timelocal(0, 0, 0, 1, 0, $year);
my $w = (localtime($t))[6];
$w = ($w – 2) % 7 + 1;
my $wc = $week * 7 – $w;
return strftime($pattern, localtime($t + $wc * 24 * 3600));
}
my $week = shift || 1;
my $year = shift || (localtime)[5];
my $pattern = shift || ‘%d/%m’;
my $t = timelocal(0, 0, 0, 1, 0, $year);
my $w = (localtime($t))[6];
$w = ($w – 2) % 7 + 1;
my $wc = $week * 7 – $w;
return strftime($pattern, localtime($t + $wc * 24 * 3600));
}
Don’t forget to add this in your script/module:
use Time::Local qw(timelocal);
use POSIX qw(strftime);
use POSIX qw(strftime);
Note: this approach works correctly if the week starts from Monday!
Update: But today I found that PostgreSQL can trancate date by week and return the first day of the week:
SELECT date_trunc(‘week’, TIMESTAMP ’2006-06-01 10:38:40′)::date
Result:
2006-05-29
So, now I can group by this date instead of week number!
Categories: Software Development
Date::Calc has:
($y, $m, $d) = Monday_of_Week($week, $year);
99% of code already written
You need to just a find it … I couldn’t do it 
Calendar cal = Calendar.getInstance();
cal.set(YEAR, year);
cal.set(WEEK_OF_YEAR, weekNo);
return new SimpleDateFormat(pattern).format(cal.getTime());
}
A bit more flexible than Monday_of_Week($week, $year), because you can customise Calendar in any way, but more verbose too.
Yes, you’re right. Basically, your method is the same as our
. But today I found that PostgreSQL can trancate date by week and return the first day of the week:
Result: