So I ended up using Date::Manip since we wanted to easily correct for holidays, weekends, etc.

use strict;
use Date::Manip;

&Date_Init("PersonalCNF=holidays.cnf","TZ=PST8PDT");

my $now = &ParseDate("now");

my $date = &ParseDate($ARGV[0]);
$date = &Date_NextWorkDay(&ParseDate($date),0) unless $ARGV[1]; 
# Anything past the first argument acts as an "override"

print &UnixDate($date,"Sleeping until %T on %F.\n");

my $delta = &DateCalc($now,$date);
my $secs = &Delta_Format($delta,0,"%sh");
if ($secs < 0) { print "Hey, that's in the past--you can't fool me!\n"
+; exit; }

print "(that's $secs seconds...)\n";
sleep $secs;
[download]

So now one can feed it nearly any date/time format with the following caveats: Days of the week are considered to be days of the *current* week, so you should always use "next <day of week>". Also, it will automagically skip holidays/weekends unless you specify something (anything) as other arguments (i.e., scalar @ARGV > 1).

The reference to holidays.cnf is a customized Date::Manip configuration file with our business holidays in it.

As a little test matrix, I ran the following (with the actual sleeping part commented out, naturally):

C:\sleep>for /F "tokens=*" %x in (testmatrix.txt) do sleepuntil %x

C:\sleep>sleepuntil "8:00 next monday"
Sleeping until 08:00:00 on Monday, October  9, 2000.
(that's 228992 seconds...)

C:\sleep>sleepuntil "14:36:24 tomorrow"
Sleeping until 14:36:24 on Monday, October  9, 2000.
(that's 252775 seconds...)

C:\sleep>sleepuntil "tomorrow" really
Sleeping until 16:23:29 on Saturday, October  7, 2000.
(that's 86400 seconds...)

C:\sleep>sleepuntil "4th thursday in november"
Sleeping until 00:00:00 on Monday, November 27, 2000.
(that's 4433790 seconds...)

C:\sleep>sleepuntil "4th thursday in november" even if it's Thanksgiving
Sleeping until 00:00:00 on Thursday, November 23, 2000.
(that's 4088190 seconds...)

C:\sleep>sleepuntil "yesterday"
Sleeping until 16:23:31 on Thursday, October  5, 2000.
Hey, that's in the past--you can't fool me!

C:\sleep>sleepuntil "2pm Dec 1, 2006"
Sleeping until 14:00:00 on Friday, December  1, 2006.
(that's 194132189 seconds...)

...and as you can see, it works beautimously. :-)