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3.6. Day in a Week/Month/Year or Week Number
3.6.1. Problem
You have a date, either in Epoch
seconds or as distinct year, month, etc. values. You want to find out
what week of the year, day of the week, day of the month, or day of
the year that the date falls on.
3.6.2. Solution
If you have Epoch seconds, the day of the year, day of the month, and
day of the week are returned by localtime. The
week of the year is easily calculated from the day of the year (but
see the following discussion, as standards
differ).
($MONTHDAY, $WEEKDAY, $YEARDAY) = (localtime $DATE)[3,6,7];
$WEEKNUM = int($YEARDAY / 7) + 1;
If you have distinct DMYHMS values, you can either convert them to
Epoch seconds values as in Recipe 3.2 and
then use the previous solution, or else use the
Day_of_Week, Week_Number, and
Day_of_Year functions from the CPAN module
Date::Calc:
use Date::Calc qw(Day_of_Week Week_Number Day_of_Year);
# you have $year, $month, and $day
# $day is day of month, by definition.
$wday = Day_of_Week($year, $month, $day);
$wnum = Week_Number($year, $month, $day);
$dnum = Day_of_Year($year, $month, $day);
3.6.3. Discussion
The Day_of_Week, Week_Number,
and Day_of_Year functions all expect years that
haven''t had 1900 subtracted from them and months where January is 1,
not 0. The return value from Day_of_Week can be 1
through 7 (corresponding to Monday through Sunday) or 0 in case of an
error (an invalid date, for example).
use Date::Calc qw(Day_of_Week Week_Number Day_of_Week_to_Text);
$year = 1981;
$month = 6; # (June)
$day = 16;
$wday = Day_of_Week($year, $month, $day);
print "$month/$day/$year was a ", Day_of_Week_to_Text($wday), "\n";
## see comment above
$wnum = Week_Number($year, $month, $day);
print "in the $wnum week.\n";
6/16/1981 was a Tuesday
in week number 25.
The governing standard
bodies of particular countries may have rules about when the first
week of the year starts. For example, in Norway the first week must
have at least 4 days in it (and weeks start on Mondays). If January 1
falls on a week with 3 or fewer days, it is counted as week 52 (or
53) of the previous year. In America, the first Monday of the year is
usually the start of the first workweek. Given such rules, you may
have to write your own algorithm, or at least look at the
%G, %L, %W,
and %U formats to the UnixDate
function in Date::Manip.
3.6.4. See Also
The localtime function in
perlfunc(1) and Chapter 29 of
Programming Perl; the documentation for the
CPAN module Date::Calc
