Fixed default reference_datetime in the System class
Fixed default reference_datetime in the System class

Previously it was constant, the value was time at import of the module.

OpenSCAP/openscap-daemon,pthierry38/openscap-daemon,OpenSCAP/openscap-daemon,pthierry38/openscap-daemon,baude/openscap-daemon,baude/openscap-daemon