cat /proc/uptime | awk '{print $1}'

$ uptime
12:13pm  up 227 day(s), 15:14,  1 user,  load average: 0.05, 0.05, 0.05

#include <utmpx.h>
int main ( )
{
  int nBootTime = 0;
  int nCurrentTime = time ( NULL );
  struct utmpx * ent;

  while ( ( ent = getutxent ( ) ) ) {
    if ( !strcmp ( "system boot", ent->ut_line ) ) {
      nBootTime = ent->ut_tv.tv_sec;
    }
  }
  printf ( "System was booted %d seconds agon", nCurrentTime - nBootTime );
}

days=`uptime | awk '{print $3}'`
hrs=`uptime | awk '{print $5}' | sed 's/[:,]/ /g' | awk '{print $1}'`
mins=`uptime | awk '{print $5}' | sed 's/[:,]/ /g' | awk '{print $2}'`
uptimesecs=$(($mins*60))
uptimesecs=$(($hrs*3600+$uptimesecs))
uptimesecs=$(($days*86400+$uptimesecs))
echo "$uptimesecs seconds uptime (to within 59 secs)."

#!/bin/ksh
now=$(nawk 'BEGIN{print srand()}')
i=$(apptrace -fv 'getutxent' uptime 2>&1 | grep time_t | tail +2 | head -1 | nawk '{print $3}')
ut=$(bc <<-%EOF%
ibase=16
$(print $i | sed 's/0x//' | tr "[a-f]" "[A-F]")
%EOF%
)
s=$((now-ut))
h=$((s/3600))
s=$((s-(h*3600)))
m=$((s/60))
s=$((s-(m*60)))
d=$((h/24))
h=$((h-(d*24)))
printf "%d seconds - %d day(s) %02d:%02d:%02dn" $((now - ut)) $d $h $m $s

$kstat -p unix:0:system_misc:boot_time

#!/usr/bin/perl -w
use strict;

my $KSTAT = '/usr/bin/kstat -p';
my $STATISTIC = 'unix:0:system_misc:boot_time';

my $uptime = `$KSTAT $STATISTIC | awk '{print $2}'`;
sprintf "%0.2fn", (time() - $uptime);

RunTime=""
RunTime=`kstat -p unix:0:system_misc:snaptime | awk '{print $2}' | cut -d "." -f1`
echo $RunTime
RunTime=`expr $RunTime / 1`
RunDays=`expr $RunTime / 86400`
RunHours=`expr $RunTime % 86400 / 3600`
RunMinutes=`expr $RunTime % 3600 / 60`
RunSeconds=`expr $RunTime % 60`