We run several CentOS 6 and CentOS 7 systems as virtual machines using VirtualBox on Windows 7. Most of these systems have continuous up time. The Windows 7 foundation platforms have no difficulty keeping accurate system time unless they are shut down for some reason, which is rare in our environment. We generally restart the Windows 7 machines at leastweekly rather than performing a complete power up restart. The CentOS virtual machines are shut down at times, usually to switch between versions that support various applications and for updates using yum. When the virtual CentOS systems are brought up, the system time always matches the Windows 7 host system time. We have recently noticed that after several hours, the CentOS system time has started to lag behind. The time difference in a week can be as much as four days. This system time lag has not always been immediately discovered by the user resulting in some annoying file date issues. Access to some web sites have also been a problem due to the "date in the future" issue. We are now on the lookout for this system time problem and restart the virtual machine to set the current date and time. Is this a common time problem when running CentOS as a virtual machine? Any suggestions regarding the cause if this problem and how to keep the CentOS system time locked to the host platform time would be greatly appreciated. Thanks.