I am running several versions of FreeBSD under VirtualBox, on a 4-core
AMD box running FreeBSD 8.2 amd64.

Since recently, and I believe since installing 8.2, every few minutes
the time in the vbox clients jumps by exactly 20 minutes like this
(hal is the host, the v90x are the vbox clients):

[0]# rdate -p hal ; date
Thu Mar  3 19:36:27 2011
Thu Mar  3 19:36:27 CET 2011
[0]# rdate -p hal ; date
Thu Mar  3 19:44:06 2011
Thu Mar  3 19:44:06 CET 2011
[0]# rdate -p hal ; date
Thu Mar  3 20:09:14 2011
Thu Mar  3 20:29:14 CET 2011 <=== jump by 20 minutes
[0]# date ; rdate hal   
Thu Mar  3 20:29:16 CET 2011 <=== manual reset using rdate
Thu Mar  3 20:09:16 2011
[0]# date ; rdate hal
Thu Mar  3 20:43:58 CET 2011
Thu Mar  3 20:43:59 2011

or this:

[0]# rdate -p hal ; date
Thu Mar  3 20:57:44 2011
Thu Mar  3 20:57:43 CET 2011
[0]# rdate -p hal ; date
Thu Mar  3 20:58:31 2011
Thu Mar  3 21:18:30 CET 2011 <=== jump
[0]# date ; rdate hal
Thu Mar  3 21:18:34 CET 2011 <=== manual reset
Thu Mar  3 20:58:35 2011
[0]# rdate -p hal ; date
Thu Mar  3 21:00:49 2011
Thu Mar  3 21:00:49 CET 2011
[0]# rdate -p hal ; date
Thu Mar  3 21:05:31 2011
Thu Mar  3 21:25:31 CET 2011 <=== jump
[0]# date ; rdate hal   
Thu Mar  3 21:25:37 CET 2011 <=== manual reset
Thu Mar  3 21:05:37 2011
[0]# date ; rdate hal
Thu Mar  3 21:08:12 CET 2011
Thu Mar  3 21:08:13 2011

This does not seem to happen on the 7.4 client which is also running at
the same time as the two others (HEAD, 8.2).

Previously (and I believe even now), if the host was very busy, the
clients would simply lag behind the host, and I could use rdate to set
them forward again. But these exact 20 minute jumps are a completely
new experience and rather annoying.

Btw., if I remember correctly, they also accumulate: If I do not manually
reset the time, after a longer while I will have the clock 40 minutes
into the future, etc.

How-To-Repeat: Run HEAD or 8.2 in VirtualBox, observe the time.