Talked with Respironics tech support.
The bug causes err 39 Nov 1 and Dec 1.
The power off/on for The Nov 1 erroneously advances the date.
The power off/on after the Dec 1 err 39 should bring
the date back to normal.

From Respironics: Respironics Advisory: Remstar Auto E039 Error Dear Customer, Our records indicate that you have purchased a RemStar Auto CPAP between May 20, 2002 and Nov 1st 2004. It has come to our attention, that due to a software bug in this series of devices, your customer may experience a E0...

Where do I find the serial number?
got this error.
curious additional symptom is that since then the date / time
recorded on the Encore reports is 84 hours in advance of actual.
I have just erased the smart card to see if this fixes it.