Excel - how to restrict values of data validation list changing to date

We are creating a data validation list from C# code. This list will contain Month with year. How can we restrict excel from converting this list from changing to date format?

Below is the code: 

range.Validation.Add(XlDVType.xlValidateList,
                               XlDVAlertStyle.xlValidAlertInformation,
                               XlFormatConditionOperator.xlBetween,
                               flatList,
                               Type.Missing);
range.Validation.IgnoreBlank = false;
range.Validation.InCellDropdown = true;

flatList will have values "June 2016, July 2016, August 2016". Excel converts the list to "Jun-2016, Jul-2016, Aug-2016". We want that to be avoided and the list should remain as text. 

Please advice.