Knowledge Is Power

Use A Different Time Zone Inside A Perl Script

Scenario

You have your Web site hosted on a server with a provider who sets the server times to UTC (aka GMT or Zulu) time and you want to run CGI scripts that use time functions. The problem is UTC time doesn't change for Daylight Saving Time so you need to use your local time zone rather than the server-based UTC time zone.

Luckily there's a very easy fix if Perl is appropriately configured on the Web server. The time zone the server is using is stored in an enviroment variable called TZ so you need to change that environment variable before you call the localtime() function. Simply add the following statements near the top of your script:

# use POSIX qw(tzset);

$ENV{TZ} = 'America/New_York';  # TZ names @ https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
# tzset;                        # Should not needed as it'll be run automatically with localtime() below

                                # The localtime() function must come AFTER the above statements
($sec,$min,$hour,$day,$month,$year,$dayofweek,$dayofyear,$isDST) = localtime(time);

Note that the tzset and use commands are commented out. This is because on POSIX-compliant systems the localtime function that follows should run tzset automatically. If using just the $ENV statement doesn't reset the system timezone to your time zone while the script is executing, try uncommenting the tzset and use commands and see if that rectifies the problem.

NOTE:  If the server timezone is set to UTC, the $isDST variable above will always be 0 (zero).

If uncommenting the tzset and use commands don't rectify the problem it's likely your Web server isn't configured appropriately. You can try contacting technical support at your Web hosting provider. If that doesn't work you'll have to resort to the code below.

In the code below we simply read the server's UTC time into a variable and modify it with the offset values for Standard and Daylight Saving times in your time zone. If you're unsure of what offset values to use, check the "UTC Offset" columns on this Wikipedia page:
https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
Just add the code given below near the beginning of your script. Note the offset values given in the script comments below are negative for US time zones and positive for European time zones.

NOTE:  Using this method means you'll have to change the offset value MANUALLY every time there is a time change.

# Perl script to offset server UTC time
# to local time.

# For STANDARD Time use:
#   Paris/Rome CET use +2 below
#   UK use +1 below
#   US Eastern Time use -5 below
#   US Central Time use -6 below
#   US Mountain Time use -7 below
#   US Pacific Time use -8 below

# For DAYLIGHT SAVING Time use:
#   Paris/Rome CET use +1 below
#   UK use 0 below
#   US Eastern Time use -4 below
#   US Central Time use -5 below
#   US Mountain Time use -6 below
#   US Pacific Time use -7 below

   #--- Read current server date/time values into variables
($sec,$min,$hour,$day,$month,$year,$dayofweek,$dayofyear,$isDST) = localtime(time);

$hour = $hour -5;  # EDIT THIS OFFSET VALUE  (now set to US Eastern STANDARD Time)

if ($hour < 0)
{
 $hour = $hour + 24;
 $day = day - 1;
 $dayofweek = $dayofweek - 1;
 $dayofyear = $dayofyear - 1;
}

Then you can use these adjusted variables ($hour $day $dayofweek $dayofyear) throughout your script. Note that the reason you want to define and initialize these variables near the top of your script is so they're defined globally for use throughout your script.

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