diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1215,7 +1215,10 @@ def matchdate(date):
         return parsedate(date, extendeddateformats, d)[0]
 
     date = date.strip()
-    if date[0] == "<":
+
+    if not date:
+        raise Abort(_("dates cannot consist entirely of whitespace"))
+    elif date[0] == "<":
         when = upper(date[1:])
         return lambda x: x <= when
     elif date[0] == ">":
diff --git a/tests/test-log.t b/tests/test-log.t
--- a/tests/test-log.t
+++ b/tests/test-log.t
@@ -512,7 +512,11 @@ log -k r1
   date:        Thu Jan 01 00:00:01 1970 +0000
   summary:     r1
   
+log -d " " (whitespaces only)
 
+  $ hg log -d " "
+  abort: dates cannot consist entirely of whitespace
+  [255]
 
 log -d -1