When adding new books if the timestamp is specified as undefined use …

…the current timestamp. Fixes #2069494 ['Sometimes' timestamp column has incorrect data](https://bugs.launchpad.net/calibre/+bug/2069494)

src/calibre/db/cache.py

@@ -45,7 +45,7 @@
 from calibre.ebooks.metadata.opf2 import metadata_to_opf
 from calibre.ptempfile import PersistentTemporaryFile, SpooledTemporaryFile, base_dir
 from calibre.utils.config import prefs, tweaks
-from calibre.utils.date import UNDEFINED_DATE, timestampfromdt, utcnow
+from calibre.utils.date import UNDEFINED_DATE, is_date_undefined, timestampfromdt, utcnow
 from calibre.utils.date import now as nowf
 from calibre.utils.filenames import make_long_path_useable
 from calibre.utils.icu import lower as icu_lower
@@ -2146,7 +2146,7 @@ def create_book_entry(self, mi, cover=None, add_duplicates=True, force_id=None,
         book_id = self.backend.last_insert_rowid()
         self.event_dispatcher(EventType.book_created, book_id)
 
-        mi.timestamp = utcnow() if mi.timestamp is None else mi.timestamp
+        mi.timestamp = utcnow() if (mi.timestamp is None or is_date_undefined(mi.timestamp)) else mi.timestamp
         mi.pubdate = UNDEFINED_DATE if mi.pubdate is None else mi.pubdate
         if cover is not None:
             mi.cover, mi.cover_data = None, (None, cover)