The server that Larry specifies for rdate timed out. So I uninstalled that and yum installed ntpdate and the same server (time.nist.gov) worked for ntpdate.
This advanced the datetime to the current (correct) datetime.
As I suspected, this confused Ambari — all graphs were flat — and then I saw that HBase went from green to red status.
From the services tab in Ambari I saw that the Stop button was de-activated for HBase. So I clicked the Start button and after a period of apparent ramp-up time, all of the conditions leading to HBase being red were corrected and HBase (and the rest of the Hadoop components) were all green again.
PLUS, the little summary graphs (for example in the Metrics section when viewing HBase) that previously were empty with No Data messages now all show real data in them.
I complained about this in another thread in this forum.
So the key takeaway is that ntpdate can sync your clock for you, but be prepared for consequences if you do.