If you are planning on using the new NAV 2009 Web services capabilities as a means of running scheduled tasks, you must ensure that the Windows user account that is to be used to execute the task has a User Personalisation record with the correct Language ID.
5129 is the English (New Zealand) language ID and If you do not have a record, or you use the English (United States) setting then you will find date fields are validated using the US M/dd/YYYY date format. This is likely to cause you a problem if you are using the Web service to process an interface file in which you are validating a date field (such as Posting Date) with a value from a text file.
I found this error, when my overnight interface processing routine was returning the error message “13/7/2009 is not a valid Posting Date”. If you are American, then you are used to your dates being backwards and will therefore be OK.