You can use the TZ Environment Variable to do this ... (detailed at the bottom of the linked page)

$ perl -e '$ENV{TZ}="EST5EDT"; print scalar(localtime),"\n";'
Mon Sep 27 15:13:13 2004
$ perl -e '$ENV{TZ}="GMT"; print scalar(localtime),"\n";'    
Mon Sep 27 19:13:22 2004
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That's a really neat trick. I can't use it though. Other applications and scripts run on the box that mine does. If I modify the environment, I'll mess up the other programs.

In any case, changing %ENV only affects perl and it's children, so your other applications and users won't be affected. (Note: perl is in the same process as Apache in mod_perl, so it would probably affect it a bit more in that scenario.)

But you use local to tightly scope the change if you were so inclined:

print scalar(localtime),"\n";
{
   local $ENV{'TZ'} = 'AST4ADT';
   print scalar(localtime),"\n";
}
print scalar(localtime),"\n";

__END__
output
======
Mon Sep 27 15:43:45 2004
Mon Sep 27 16:43:45 2004
Mon Sep 27 15:43:45 2004
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All the *good* answers to this question are going to involve some module or another. Otherwise, you're going to have to basically duplicate the (relevant) functionality of the module in your code.

The module doesn't have to be Date::Calc necessarily. There are numerous other options. (I would use DateTime myself.) You *may* even be able to find one that's pure Perl and doesn't have a lot of dependencies, so that you can just place a copy of the .pm in the same directory as your code and not have to install it per se. (DateTime is not the answer in this respect; it has a trainload of dependencies; the reason I use it is because it does everything. The DateTime::Format modules are particularly useful. But that stuff is beyond your needs just at the moment.) Poke around on search.cpan.org with search strings like "date" and "time" and "timezone" and see if you can find a _simple_ module that's pure perl and just does what you want.

I don't know of a way w/o Date::Calc, however depending on what you are doing you may not need perl. If you are doing something as simple as taking the users local time and displaying a msg like good morning, good afternoon, good evening, etc. That is better left to stuff like Javascript. However if it's not (which i suspect it isn't) as simple as that then i'm inclined to ask why you can't install Date::Calc? just restrictions or not wanting to fool with it?

Thanks Grygonos. I need to do it in Perl. Based on a time, my script needs to do differnt things. I can't install CPAN modules becuase the environment that this code is in must use Perl 5.6.1 with no other modules.

Yes that is correct. There is always a 5 hour difference between GMT and EST and between BST and EDT. That does't help when GMT switches to BST a week before EST switches to EDT. That's exactly why I can't hard code +5 hours. The link below show the dates that DST start and end in NY and LDN.

http://www.timeanddate.com/worldclock/city.html?n=179
http://www.timeanddate.com/worldclock/city.html?n=136

If you care about the time difference between two places, then ask for that. Instead, you asked:

If I'm using a machine that is on GMT time, how can I convert that time to EST using a standard Perl 5.6.1 installation?

And the answer is "subtract five hours". Always.