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Full Discussion: Find Day of Week
Operating Systems HP-UX Find Day of Week Post 302731167 by rwuerth on Wednesday 14th of November 2012 10:54:49 AM
Old 11-14-2012
Find Day of Week

In HP-UX the date command does not have the "-d" switch like some other *nixes do. I'm working a simple script to tell me, given the day, month and year what day of the week that falls on.

Assuming valid day, month and year input (I'd perform quality checks on the input separately, but not shown here), would the following work for all dates? Or are there some corner cases that may not work correctly? I'm not worried about corner cases like way back in the 1500's when the Gregorian calendar 1st came online and how that might be screwed up.

The output is the output from a call to the 'cal' utility, plus the full name of the day of the week so that the named day of the week can be easily verified.

Code:
#!/bin/sh
day=$1
mth=$2
yr=$3
 
cal $mth $yr
 
set -A _WKDY Saturday Sunday Monday Tuesday Wednesday Thursday Friday
 
print ${_WKDY[$(cal $mth $yr | awk -vDD=$day 'FNR==3 {print ((7-$NF)+DD)%7}')]}

in awk,

Code:
FNR==3

means I skip the headers and am only concerned with the first week of the 'cal' output.

Breaking the math down:

Code:
(7-$NF)

Is an offset to "normalize" partial weeks since the first week of a month may have less than 7 days in it.

Code:
((7-$NF)+DD)

The offset is added to the numerical day of the month (DD)

Code:
((7-$NF)+DD)%7

The offset plus DD is then modded by 7 giving the numerical day of the week, of which, 0 is defined as Saturday, and 1-6 is defined as Sunday through Friday by how I set up the array.

I'm basically just wondering if anyone can break this given a modern date and valid inputs. (no using February 30th! :-) ) I can't break it, but I've a nagging suspicion I'm missing something. Smilie

TIA
 

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Time::CTime(3pm)					User Contributed Perl Documentation					  Time::CTime(3pm)

NAME
Time::CTime -- format times ala POSIX asctime SYNOPSIS
use Time::CTime print ctime(time); print asctime(localtime(time)); print strftime(template, localtime(time)); strftime conversions %% PERCENT %a day of the week abbr %A day of the week %b month abbr %B month %c ctime format: Sat Nov 19 21:05:57 1994 %d DD %D MM/DD/YY %e numeric day of the month %f floating point seconds (milliseconds): .314 %F floating point seconds (microseconds): .314159 %h month abbr %H hour, 24 hour clock, leading 0's) %I hour, 12 hour clock, leading 0's) %j day of the year %k hour %l hour, 12 hour clock %m month number, starting with 1, leading 0's %M minute, leading 0's %n NEWLINE %o ornate day of month -- "1st", "2nd", "25th", etc. %p AM or PM %r time format: 09:05:57 PM %R time format: 21:05 %S seconds, leading 0's %t TAB %T time format: 21:05:57 %U week number, Sunday as first day of week %v DD-Mon-Year %w day of the week, numerically, Sunday == 0 %W week number, Monday as first day of week %x date format: 11/19/94 %X time format: 21:05:57 %y year (2 digits) %Y year (4 digits) %Z timezone in ascii. eg: PST DESCRIPTION
This module provides routines to format dates. They correspond to the libc routines. &strftime() supports a pretty good set of coversions -- more than most C libraries. strftime supports a pretty good set of conversions. The POSIX module has very similar functionality. You should consider using it instead if you do not have allergic reactions to system libraries. GENESIS
Written by David Muir Sharnoff <muir@idiom.org>. The starting point for this package was a posting by Paul Foley <paul@ascent.com> LICENSE
Copyright (C) 1996-2010 David Muir Sharnoff. Copyright (C) 2011 Google, Inc. License hereby granted for anyone to use, modify or redistribute this module at their own risk. Please feed useful changes back to cpan@dave.sharnoff.org. perl v5.12.3 2011-05-12 Time::CTime(3pm)
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