ExifTool, which PhotoStructure uses behind the scenes to gather metadata, handles date time stamps with 0 values for months, days, and hours, presumably to represent “unknown” values. PhotoStructure should handle these unset fields gracefully.
These unset fields are useful for scanned images where exact date and time are unknown.
In addition to scanned images, this would cover images that were captured by a camera, with what is known to be incorrect date and time, for which accurate date is unknown, and thus that timestamp has been set in metadata to 0.