How do I set the time and date?

Assuming you're running under sufficient permissions, you should be to set the system-wide date and time by running the date program. (There is no way to set the time and date on a per-process basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; the VMS equivalent is set time.

However, if all you want to do is change your timezone, you can probably get away with setting an environment variable: 

    $ENV{TZ} = "MST7MDT";		   # unixish
    $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
    system "trn comp.lang.perl";