The date format can be specified as a default or be different for each target.  The date format is for the %LogTime% variable in the MessageFormat.  The default is: yyyy-MM-dd HH:mm:ss,fff  Any .NET Date Formatting can be used.  See http://msdn.microsoft.com/en-us/library/az4se3k1.aspx and  http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx

//Specify it as a default
Log.Config.DefaultDateFormat = "yyyy-MM-dd HH:mm:ss,fff";

//Override it for a specific target
StringBuilderTarget target = new StringBuilderTarget();
target.DateFormat = "g";
Log.Config.Targets.Add(target);