An empty cell returns a value of null.

Setting Value to null or to an empty string clears the value from the cell.

Setting Value to a string has the same affect as setting Formula to the same string, the only difference being that setting Formula directly is faster. See Formula for more information on the parsing of string values to numbers, dates, times, errors, logical values and text.

Dates and times are stored internally as a System.Double. Setting Value to a System.DateTime converts the date and/or time to a System.Double. Use IWorkbook.DateTimeToNumber and IWorkbook.NumberToDateTime to convert from a System.DateTime to a number suitable for a cell and back.

The Value property returns the raw data of a cell. Use Formula to get the formula of a cell or use Text to get the formatted value of a cell.

Reference