VirtualBox Ubuntu VM doesn't automatically sync time with host even with Guest Additions installed

Solution 1:

Turns out I needed to install the ntp package, in addition to the two others I mentioned above.

The date/time after I woke my computer up from sleep:

$ date
Wed Sep 25 12:50:25 EDT 2013

Then, to correct it:

$ sudo service ntp stop
 * Stopping NTP server ntpd                          
$ sudo ntpdate pool.ntp.org
25 Sep 13:22:45 ntpdate[15379]: step time server 109.169.89.48 offset 1880.691920 sec
$ sudo service ntp start
[sudo] password for yiqing: 
 * Starting NTP server ntpd  

Now synced to the real date/time:

$ date
Wed Sep 25 13:23:07 EDT 2013

Once I rebooted, I found that my VM's system clock synced to my host as desired.