Reports always blank when period is specified

I can never get Reports to work when I specify a period between two dates.

If I enter the two dates, it just produces a blank report.

If I leave the period from/to fields empty it does produces a report - but for the entire period the router has been gathering data.

I had thought this might be due to erroneous dates in the database, related to NTP because I have been using remote NTP servers. For instance if the device boots up after a power outage and is active before the gateway, and someone logs on it might not be able to synch the clock in time and record a bad start time (I see sessions with year 1970 when this happens).

Can anyone point to possible causes, thanks.