By default, CentOS/RHEL dmesg timestamps are disabled. Here are the instructions to enable timestamps and a quick script to display time in a human-readable format.

echo Y | sudo tee /sys/module/printk/parameters/time
echo "Enabled timestamps" | sudo tee /dev/kmsg

To make this change survive reboot, update /etc/rc.d/rc.local as shown below

cat << EOF >> /etc/rc.d/rc.local
echo 1 > /sys/module/printk/parameters/printk_time
EOF

And the script:

#!/bin/bash
date_format="%a %b %d %T %Y"
uptime=$(cut -d " " -f 1 /proc/uptime)
if [ "Y" = "$(cat /sys/module/printk/parameters/time)" ]
then
	dmesg | sed "s/^\[[ ]*\?\([0-9.]*\)\] \(.*\)/\1 \2/" | while read timestamp message
	do
		printf "[%s] %s\n" "$(date --date "now - $uptime seconds + $timestamp seconds" +"${date_format}" 2>/dev/null)" "$message"
	done
else
	echo "Timestamps are disabled (/sys/module/printk/parameters/time)"
fi

Save it as /var/adm/bin/dmesgt.sh; make it executable; and create a convenient link:

chmod 755 /var/adm/bin/dmesgt.sh
ln -s /var/adm/bin/dmesgt.sh /usr/bin/dmesgt

Result:

# dmesgt | tail -1
[Fri Jun 12 16:57:00 2015] Enabled timestamps