Issue is discussed in detail on the Adafruit forum here:
https://forums.adafruit.com/viewtopic.php?t=204255

The datetime can be parsed from the serial data returned by the GPS module, but as it is marked as invalid (V-marker?) due to having no GPS data, it is ignored by the library. The suggested improvement would be to examine the contents of the data whenever datetime is requested to see if there is a valid datetime, regardless of the validity marking.

Current workaround is to manually parse the serial data within the code, rather than relying on the library.