The publish at and unpublish at functionality is not currently working on my site which is running Umbraco 7.1.4 if I view the UmbracoTraceLog.txt file in the App_Data/Logs folder I see the following error showing up every minute.

Any ideas what the cause might be??

2015-07-20 15:01:55,679 [38] ERROR umbraco.presentation.publishingService - [Thread 7] Error executing scheduled publishing
System.Data.SqlClient.SqlException (0x80131904): The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryHasMoreRows(Boolean& moreRows)
   at System.Data.SqlClient.SqlDataReader.TryReadInternal(Boolean setTimeout, Boolean& more)
   at System.Data.SqlClient.SqlDataReader.Read()
   at Umbraco.Core.Persistence.Database.<Query>d__1c`1.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Umbraco.Core.Persistence.Repositories.ContentRepository.<PerformGetByQuery>d__24.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at umbraco.presentation.publishingService.CheckPublishing(Object sender)
ClientConnectionId:5838da32-95d5-4103-9554-399f4775cdcc