diff --git a/src/calibre/utils/date.py b/src/calibre/utils/date.py
index 4b9ed62bbc..5e10598baf 100644
--- a/src/calibre/utils/date.py
+++ b/src/calibre/utils/date.py
@@ -12,7 +12,7 @@ from functools import partial
 
 from calibre import strftime
 from calibre.constants import iswindows, isosx, plugins
-from calibre.utils.iso8601 import utc_tz, local_tz
+from calibre.utils.iso8601 import utc_tz, local_tz, UNDEFINED_DATE
 from calibre.utils.localization import lcdata
 
 _utc_tz = utc_tz
@@ -53,7 +53,6 @@ else:
     except:
         parse_date_day_first = False
 
-UNDEFINED_DATE = datetime(101,1,1, tzinfo=utc_tz)
 DEFAULT_DATE = datetime(2000,1,1, tzinfo=utc_tz)
 EPOCH = datetime(1970, 1, 1, tzinfo=_utc_tz)
 
diff --git a/src/calibre/utils/iso8601.py b/src/calibre/utils/iso8601.py
index 87fba239ea..a425056f45 100644
--- a/src/calibre/utils/iso8601.py
+++ b/src/calibre/utils/iso8601.py
@@ -29,9 +29,12 @@ class SafeLocalTimeZone(tzlocal):
 utc_tz = tzutc()
 local_tz = SafeLocalTimeZone()
 del tzutc, tzlocal
+UNDEFINED_DATE = datetime(101,1,1, tzinfo=utc_tz)
 
 if hasattr(speedup, 'parse_iso8601'):  # For people running from source without updated binaries
-    def parse_iso8601(date_string, assume_utc=True, as_utc=True):
+    def parse_iso8601(date_string, assume_utc=False, as_utc=True):
+        if not date_string:
+            return UNDEFINED_DATE
         dt, aware, tzseconds = speedup.parse_iso8601(date_string)
         tz = utc_tz if assume_utc else local_tz
         if aware:  # timezone was specified
@@ -133,7 +136,9 @@ else:
             minutes = -minutes
         return tzoffset(description, 3600*hours + 60*minutes)
 
-    def parse_iso8601(date_string, assume_utc=True, as_utc=True):
+    def parse_iso8601(date_string, assume_utc=False, as_utc=True):
+        if not date_string:
+            return UNDEFINED_DATE
         if isinstance(date_string, bytes):
             date_string = date_string.decode('ascii')
         m = iso_pat().match(date_string)