This appears to have been a firmware bug resolved at the 18.104.22.168 level (which correlates with your <datetime> outputs).
See the CP-X4 Firmware Release Notes file for additional details:
82001536_K (22.214.171.124) - October 14, 2011
The clock (time) source management functionality has been improved to better detect failures to retrieve time sync samples from an NTP server as a time source. Failures or "lost" replies result in quick retries for both the initial sample after boot time as well as for subsequent samples. Additional configurability is supported via the "set timemgmt" CLI command, the web UI and the iDigi platform. Event logging is improved for time-related events.
For the clock (time) source management feature, update the web UI and web help to include the configurable jump threshold and "lost time source" detection settings.
For the clock (time) source management feature, improve the SNTP client implementation:
- Add detection of "expired" NTP replies.
o Accept replies only if in response to the most recently issued request for a given NTP time source. Discard other replies.
o Replies must not be more than 20 seconds since the request was sent. Old samples can skew the time computation and cause undesired jumps.
o Add a statistical counter for "expired" replies ("info time" CLI).
- Improve the NTP reply read resolution for the socket, to more quickly process replies.