My Guest OS slide local time every days

hi everyone

My system information

Vmware Version : VMware-VMvisor-Installer-5.1.0-799733 Build (when i downloaded in the dell web site)

Hardware Model : dell R720 Model

issue : My Guest OS(Redhat 6.2 version) slide local time every days (All Guest OS)

but we don`t makes a NTP server (environment, policy, etc...)

so i have done time sync static myself (Host,Guest OS) and then Guest OS(window,Redhat) slide local time on next day when i check the systems

also i have done timesync with host and then i was mornitering all of my Guest os but slide Maybe 15 hour after 5 minute

(example local time 10AM -> tomorrow 1AM)

so i`m very confuse if i can not config ntp server what can i do and check vsphere and guest os

thank you