This happens on a CSV Excel file where a value initially formatted as Text is converted by Excel to the Date format when the file is saved and evenutually reopenned.
1. Open MS Excel.
2. In cell A1 using your mouse right click > Format Cells > Text > Click OK.
3. Input Sept 2013
4. Save the file as CSV (Comma delimited) type and reopen it.
Result: Cell A1 now shows Sep-13 or 9/1/2013 in a custom format mmm-yy. If the CSV file is re-saved it will retain the incorrect format and an invalid reference error will be received if the same is used on a CSV import.
To stop MS Excel from automatically converting the value from Text to a Date format put an equal sign with a double quote on the value. It will force the data to use the Text format.
In the above example Sept 2013 should be keyed as =" Sept 2013". Note that there is a space in between the double quote " and the value Sept.