Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I need to convert time from GMT to Chicago local time. I tried the following but it looks broken. 
sub fromGMT { my ($date, $hms) = @_;  
  my ($Y, $M, $D) = unpack('A4A2A2',$date);  
  my ($H, $M, $S) = $hms =~ m/^(\d+):(\d+):?(\d+)?/; $S ||= 0;  
   $dt=DateTime->new(year=>$Y, month=>$M, day=>$D, hour=>$H,minute=>$M
+,second=>$S,time_zone=>'GMT');  
   $dt->set_time_zone('America/Chicago'); 
   $dt->is_dst() and $dt->add( hours=>1 );   # Unneeded
   return $dt->hms; 
}

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Re: GMT to Chicago TIme
by ikegami (Patriarch) on May 21, 2010 at 21:06 UTC
    Use use strict; use warnings;. You are hiding valuable error messages if you don't. Specifically, 
    "my" variable $M masks earlier declaration in same scope at a.pl line 
+8.
Global symbol "$dt" requires explicit package name at a.pl line 9.
Global symbol "$dt" requires explicit package name at a.pl line 10.
Global symbol "$dt" requires explicit package name at a.pl line 12.
Execution of a.pl aborted due to compilation errors.

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    Fix: 

    sub fromGMT { my ($date, $hms) = @_;
   my ($Y, $m, $d) = unpack('A4A2A2',$date);  
   my ($H, $M, $S) = $hms =~ m/^(\d+):(\d+)(?::(\d+))?/; $S ||= 0;  
   my $dt = DateTime->new(
      year=>$Y, month=>$m,  day=>$d,
      hour=>$H, minute=>$M, second=>$S,
      time_zone=>'GMT'
   );
   $dt->set_time_zone('America/Chicago'); 
   return $dt->hms;
}

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Re: GMT to Chicago TIme
by Marshall (Canon) on May 22, 2010 at 01:11 UTC
    I need to convert time from GMT to Chicago local time.

    Well, right now in CDST (Central Daylight Standard Time), Add 5 hours to local time to get the time in Greenwich England. When Chicago "goes off daylight savings time", (like the saying: "fall back"), the time difference will be 6 hours instead of 5 hours.

    There are various Perl modules that calculate date/times. But basically they all involve a "trip through GMT". GMT in a Unix sense, is a continuously incrementing number of seconds. CST or CDST is a presentation of GMT. 1:30 AM local time could possibly mean 2 different GMT times for the same local day/time ( the "spring forward, fall back" idea). What "local time" is, is a political decision.

    There is a boundary condition. But essentially the date/time module will know whether to subtract 5hrs*60min*60sec or 6hrs*60min*60sec from Unix "GMT epoch seconds" to get local time. Hard code this or ask a module that has a continuously updated database.

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      I don't understand your point. The module he is using is quite aware as to when Chicago uses DST and when it doesn't. 
      print(fromGMT('20090101', '12:00:00'), "\n");  # 06:00:00
print(fromGMT('20090701', '12:00:00'), "\n");  # 07:00:00

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        My brain was elsewhere on this one...burning a 7 DVD set for one project while working on another coding project. Shouldn't have even been adding a 3rd thing to the multi-tasking list by answering a Perl question. I tried to do too much at once and quality of the result is less than I would have desired. Sorry. My oops.
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