SqlBulkCopy Converting string to datetime with US Format

I'm transferring date from an unknown source (Could be csv/SQL Server) to an application database, As i don't know where the data is coming from I first import the data into a holding table (all Varchar Columns) I then test is the value in the column matches what it should be and import to the database or flag a data error

To transfer the data i load the data into a DataTable in C# then use the SqlBulkCopy to insert the data to the holding table.This is all working but i'm getting an issue with datetime columns. In the C# DataTable the date value is in YMD format and is stored as a string, the column in the holing table is a varchar(255) but at some point the transfer process the date is being converted to DMY format this is then causing an issue when I try to validate the dates in the holding table.

To Load the DataTable I do:

cmd.CommandText = string.Format("SET DATEFORMAT YMD; {0}", cmd.CommandText);

SqlDataAdapter dataAdapter = new SqlDataAdapter(cmd);
DataTable returnTable = new DataTable();

dataAdapter.Fill(returnTable);

To Transfer the data I do:

    using (SqlBulkCopy bulkCopy = new SqlBulkCopy(DBContext.ConnectionString, SqlBulkCopyOptions.KeepNulls))
    {
        bulkCopy.DestinationTableName = "HoldingTable";
        bulkCopy.BatchSize = 10000;

        foreach (impMap im in ImportMapping.Where(i => i.IsImported))
        {
            bulkCopy.ColumnMappings.Add(im.MappingColumn, im.TableField.FieldName);
        }

        foreach (SqlBulkCopyColumnMapping m in columnMappings)
        {
          bulkCopy.ColumnMappings.Add(m);
        }

        bulkCopy.WriteToServer(dataDataTable);  
   }