A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The format of a column does not determine the type of the data in it. If you have text in a column, you can apply any format you like to the column, and the content will still be text. If you change the format of column C, does the appearance of the cells change at all? I suspect not, in which case they are text and that would explain what you are describing. If the text dates are in a format that matches your regional settings, you could simply add +0 to the end of the reference to column C to convert them to real dates.