Age of a process in seconds on the Linux proc system

pid_age.pl

#!/usr/bin/perl

$pid = shift || die "You must specify a pid\n";

die "Pid $pid does not exist\n" unless -e "/proc/${pid}/";

my $s = `cat /proc/${pid}/stat`;
my @a = split( /\s+/, $s );
my $jiffies = int( $a[22] / 1000 );

my $u = `cat /proc/uptime`;
my @ua = split( /\s+/, $u );
my $uptime = int( $ua[0] );
my $now = time();
my $launched = $now - $uptime + $jiffies;
my $age = $now - $launched;

print "Age: $age\n";