#!/bin/sh
# ago.shl
# usage: ago.shl [number] 
#       for date [number] days ago, at midnight, last second of that day.
#       ./ago.shl 3 /path/to/myfile
#   parms $1 days in past 
#         $2 name of  file to change mtime on
ago()
{
   perl -e ' my $delta = $ARGV[0];
             $delta*=86400;
             $delta=time - $delta;   #  subtract delta from "now"
              ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                            localtime($delta);
             
             printf("%04d%02d%02d%02d%02d.%02d\n",
                     $year+1900, $mon+1, $mday, 23, 59, 59);
             ' $1
}

# ago 1  last second of yesterday i.e., 2017062359.59 in touch format

when=$(ago $1)
# echo "$when"
touch -t "$when" $2
exit 0