diff --git a/hgext/blackbox.py b/hgext/blackbox.py
--- a/hgext/blackbox.py
+++ b/hgext/blackbox.py
@@ -33,6 +33,11 @@ Examples::
   # rotate up to N log files when the current one gets too big
   maxfiles = 3
 
+  [blackbox]
+  # Include nanoseconds in log entries with %f (see Python function
+  # datetime.datetime.strftime)
+  date-format = '%Y-%m-%d @ %H:%M:%S.%f'
+
 """
 
 from __future__ import absolute_import
@@ -82,6 +87,9 @@ configitem('blackbox', 'maxfiles',
 configitem('blackbox', 'track',
     default=lambda: ['*'],
 )
+configitem('blackbox', 'date-format',
+    default='%Y/%m/%d %H:%M:%S',
+)
 
 lastui = None
 
@@ -169,7 +177,8 @@ def wrapui(ui):
                 return
             ui._bbinlog = True
             default = self.configdate('devel', 'default-date')
-            date = dateutil.datestr(default, '%Y/%m/%d %H:%M:%S')
+            date = dateutil.datestr(default,
+                                    ui.config('blackbox', 'date-format'))
             user = procutil.getuser()
             pid = '%d' % procutil.getpid()
             formattedmsg = msg[0] % msg[1:]
diff --git a/tests/test-blackbox.t b/tests/test-blackbox.t
--- a/tests/test-blackbox.t
+++ b/tests/test-blackbox.t
@@ -82,6 +82,16 @@ recursive aliases work correctly
   1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> so-confusing exited 0 after * seconds (glob)
   1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> blackbox
 
+custom date format
+  $ rm ./.hg/blackbox.log
+  $ hg --config blackbox.date-format='%Y-%m-%d @ %H:%M:%S' \
+  >    --config devel.default-date='1334347993 0' --traceback status
+  A a
+  $ hg blackbox
+  2012-04-13 @ 20:13:13 bob @0000000000000000000000000000000000000000 (5000)> --config 'blackbox.date-format=%Y-%m-%d @ %H:%M:%S' --config 'devel.default-date=1334347993 0' --traceback status
+  2012-04-13 @ 20:13:13 bob @0000000000000000000000000000000000000000 (5000)> --config 'blackbox.date-format=%Y-%m-%d @ %H:%M:%S' --config 'devel.default-date=1334347993 0' --traceback status exited 0 after * seconds (glob)
+  1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> blackbox
+
 incoming change tracking
 
 create two heads to verify that we only see one change in the log later