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As robinsc says, all the cluster nodes run ntpd. ntpd maintains clock sync on a continuous basis. Although the exact sync distance can vary based on network congestion, CPU time starvation etc, I've rarely seen it above 0.1s in Exadata with nearby time source. You can find out your exact sync distance by running the "ntptrace" command on a database server.
As I haven't seen the text of your SR with Oracle support I'm not sure where the 15 minute number comes from, but it may be related to the cluster time synchronization service (CTSS). CTSS generally functions as a back-up system to ntpd, and takes over if ntpd can't maintain time sync for any reason.
Thanks, below was the response from Oracle.
Each machine has a different clock frequency and as a result a slightly different time drift. NTP computes this time drift every about 15 minutes, and stores this information in a "drift" file, it then adjusts the system clock based on this known drift as well as compares it to a given time-server the sys-admins sets up. This is the recommended approach.
You can find the jitter value from below command for max time diffrence.
[root@devdb01 ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
*ds1000dsp01.ds. 18.104.22.168 2 u 78 1024 377 0.210 -7.371 12.191
ds1000dsp02.ds. 10.30.64.205 3 u 117 1024 377 0.218 -0.176 7.882
Ahhh. The ntp "drift" file is an estimate of systematic skew of a local system's clock. In other words, measuring if the local clock is consistently slow or fast, and by how much. In case the server is rebooted unexpectedly, the current drift information is written to disk every 15 minutes.
The clock sync itself, however, runs independently of the drift file write and doesn't wait 15 minutes between syncs.