The AMD must be in Sync with the report server time. If you go to the AMD configuration settings on the rum console, go to general and you should see "IP Address of the server authorized to set AMD time" -- you should put the IP address of your primary CAS over here.
Ideally, the Report Servers (CAS/ADS) and the AMDs will all sync time from the same NTP server; this will give time synchronization within milliseconds.
If NTP is not available, the Report Servers will automatically push time updates to the AMDs every interval, and this has an acceptable granularity of +/- 10 seconds. The 'authorized time server' option in the AMD's global configuration generally should only be used if the Report Servers themselves have trouble keeping time synchronization with each other. One known caveat to this is that if some AMDs drift ahead and some behind the report server, you will occasionally get the >20 seconds notification using Report Server time, as this falls right on the edge of the +/-10 seconds granularity.
The AMD's host RHEL OS has NTP already built in; it normally just needs to be configured.
For the Report Servers, I personally use
This is a Windows build of the standard open source NTP package, and I have found it to be very reliable.