Domain time drifting with VM DCs / host domain members

I'm following the advice given in this article: http://technet.microsoft.com/en-us/library/virtual_active_directory_domain_controller_virtualization_hyperv(WS.10).aspx#deployment_considerations_for_virtualized_domain_controllers

However my domain is slowly losing time, its about 7 minute behind what the time servers indicate is the current time.

I have two HyperV hosts, with a DC on each one. The hosts are domain members as well.

The first HyperV host is HyperV Server 2012 R2, the DC is SBS 2011. The second HyperV host is Windows Server 2008 R2, the DC is Server 2012 (not R2).

How can I keep the domain time correct?

EDIT:

Output of w32tm /query /status /verbose

PS C:\Windows\system32> w32tm /query /status /verbose
Leap Indicator: 0(no warning)
Stratum: 2 (secondary reference - syncd by (S)NTP)
Precision: -6 (15.625ms per tick)
Root Delay: 0.0621217s
Root Dispersion: 0.0288198s
ReferenceId: 0xA6468829 (source IP:  166.70.136.41)
Last Successful Sync Time: 12/16/2014 2:18:12 PM
Source: 2.us.pool.ntp.org
Poll Interval: 10 (1024s)

Phase Offset: -0.0000639s
ClockRate: 0.0156250s
State Machine: 2 (Sync)
Time Source Flags: 0 (None)
Server Role: 576 (Reliable Time Service)
Last Sync Error: 0 (The command completed successfully.)
Time since Last Good Sync Time: 637.4983578s