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Top Forums Shell Programming and Scripting Need help with date arithmetic please Post 303000252 by jim mcnamara on Friday 7th of July 2017 11:42:00 AM
Old 07-07-2017
My opinion is that coding date arithmetic and not using existing tools is foolish and error prone. I cannot tell what system you are on, most systems have perl.

Code:
#!/bin/sh
# ago.shl
# usage: ago.shl [number] 
#       for date [number] days ago,
#       ./ago.shl -3 
#   parm $1 == days in past e.g., yesterday = -1,
#              days in future e.g., tomorrow = 1 

ago()
{
   perl -e ' my $delta = $ARGV[0];
             $delta*=86400;          #  seconds in a day
             $delta=time + $delta;   #  epoch seconds today plus ( or minus) delta
              ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                            localtime($delta);
             
             printf("%04d%02d%02d%02d%02d.%02d\n",
                     $year+1900, $mon+1, $mday;
             ' $1
}

# example usage of the function ago

when=$(ago $1)
echo "$when"

exit 0

If you are on Linux I would HIGHLY recommend that you use the date command which is very reliable and actually supports options like yesterday and tomorrow. Skip using the perl above.
 

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Time::ParseDate(3)					User Contributed Perl Documentation					Time::ParseDate(3)

NAME
Time::ParseDate -- date parsing both relative and absolute SYNOPSIS
use Time::ParseDate; $seconds_since_jan1_1970 = parsedate("12/11/94 2pm", NO_RELATIVE => 1) $seconds_since_jan1_1970 = parsedate("12/11/94 2pm", %options) OPTIONS
Date parsing can also use options. The options are as follows: FUZZY -> it's okay not to parse the entire date string NOW -> the "current" time for relative times (defaults to time()) ZONE -> local timezone (defaults to $ENV{TZ}) WHOLE -> the whole input string must be parsed GMT -> input time is assumed to be GMT, not localtime UK -> prefer UK style dates (dd/mm over mm/dd) DATE_REQUIRED -> do not default the date TIME_REQUIRED -> do not default the time NO_RELATIVE -> input time is not relative to NOW TIMEFIRST -> try parsing time before date [not default] PREFER_PAST -> when year or day of week is ambigueous, assume past PREFER_FUTURE -> when year or day of week is ambigueous, assume future SUBSECOND -> parse fraction seconds VALIDATE -> only accept normal values for HHMMSS, YYMMDD. Otherwise days like -1 might give the last day of the previous month. DATE FORMATS RECOGNIZED
Absolute date formats Dow, dd Mon yy Dow, dd Mon yyyy Dow, dd Mon dd Mon yy dd Mon yyyy Month day{st,nd,rd,th}, year Month day{st,nd,rd,th} Mon dd yyyy yyyy/mm/dd yyyy-mm-dd (usually the best date specification syntax) yyyy/mm mm/dd/yy mm/dd/yyyy mm/yy yy/mm (only if year > 12, or > 31 if UK) yy/mm/dd (only if year > 12 and day < 32, or year > 31 if UK) dd/mm/yy (only if UK, or an invalid mm/dd/yy or yy/mm/dd) dd/mm/yyyy (only if UK, or an invalid mm/dd/yyyy) dd/mm (only if UK, or an invalid mm/dd) Relative date formats: count "days" count "weeks" count "months" count "years" Dow "after next" Dow "before last" Dow (requires PREFER_PAST or PREFER_FUTURE) "next" Dow "tomorrow" "today" "yesterday" "last" dow "last week" "now" "now" "+" count units "now" "-" count units "+" count units "-" count units count units "ago" Absolute time formats: hh:mm:ss[.ddd] hh:mm hh:mm[AP]M hh[AP]M hhmmss[[AP]M] "noon" "midnight" Relative time formats: count "minutes" (count can be franctional "1.5" or "1 1/2") count "seconds" count "hours" "+" count units "+" count "-" count units "-" count count units "ago" Timezone formats: [+-]dddd GMT[+-]d+ [+-]dddd (TZN) TZN Special formats: [ d]d/Mon/yyyy:hh:mm:ss [[+-]dddd] yy/mm/dd.hh:mm DESCRIPTION
This module recognizes the above date/time formats. Usually a date and a time are specified. There are numerous options for controlling what is recognized and what is not. The return code is always the time in seconds since January 1st, 1970 or undef if it was unable to parse the time. If a timezone is specified it must be after the time. Year specifications can be tacked onto the end of absolute times. If "parsedate()" is called from array context, then it will return two elements. On sucessful parses, it will return the seconds and what remains of its input string. On unsucessful parses, it will return "undef" and an error string. EXAMPLES
$seconds = parsedate("Mon Jan 2 04:24:27 1995"); $seconds = parsedate("Tue Apr 4 00:22:12 PDT 1995"); $seconds = parsedate("04.04.95 00:22", ZONE => PDT); $seconds = parsedate("Jan 1 1999 11:23:34.578", SUBSECOND => 1); $seconds = parsedate("122212 950404", ZONE => PDT, TIMEFIRST => 1); $seconds = parsedate("+3 secs", NOW => 796978800); $seconds = parsedate("2 months", NOW => 796720932); $seconds = parsedate("last Tuesday"); $seconds = parsedate("Sunday before last"); ($seconds, $remaining) = parsedate("today is the day"); ($seconds, $error) = parsedate("today is", WHOLE=>1); AUTHOR
David Muir Sharnoff <muir@idiom.com>. LICENSE
Copyright (C) 1996-2006 David Muir Sharnoff. License hereby granted for anyone to use, modify or redistribute this module at their own risk. Please feed useful changes back to muir@idiom.com. perl v5.12.1 2006-08-15 Time::ParseDate(3)
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