The time at the top of the display is provided by the program
/etc/smgr, which combines the functionality of cron, /etc/update, a
time display, plus maybe a few other mysterious things.  Equally
mysterious is its propensity to occasionally die, leaving a stipple
pattern in its place.  

As smgr does its cron processing, it logs what it is doing in
/usr/adm/cron.log.  So that this file does not get too large,
smgr responds to signal 17 (SIGUSR2) by truncating the log file.
Unfortunately, smgr will exit if the log file is missing.

If you want to make sure this does not happen anymore, edit
/etc/cleanup.wk (the script that runs weekly and sends smgr the
clean up signal) and add
	touch /usr/adm/cleanup.wk
before the script sends the signal with "kill -17 $PID"

	Restarting smgr is thankfully unmysterious; become root and
type "/etc/smgr".