The Excel Mid function returns a specified number of characters from the middle of a supplied text string.
The format of the function is :
Where the function arguments are:
text    The original text string. 
start_num    An integer that specifies the position of the first character that you want to be returned. 
num_chars    An integer that specifies the number of characters (beginning with start_num), to be returned from the supplied text. 
The Mid function always returns a text string, even though this may be contain digits, and therefore may look like a number. This should be borne in mind if you wish to use the result of the function within further functions and formulas.
The spreadsheet below shows three examples of the Mid function.
The formulas are shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Note that the example in cell B3 returns the text value "5". Although the text string contains a number, this is stored as a text string in Excel. Excel Data Types are explained further on the Excel Formatting page.
Further information and examples of the Excel Mid function are provided on the Microsoft Office website.
Some users make the mistake of attempting to use the , Left, Mid or Right functions on dates.
Dates are stored in Excel as numbers, and it is only the cell formatting that makes them appear as dates in your spreadsheet. Therefore, if you attempt use the Left, Mid or Right function on a date, the function will return the end characters of the number that represents that date.
For example, 01/01/1980 is represented by the number 29221, so applying the Mid function to a cell containing the date 01/01/1980 (and requesting that 2 characters, starting from the 2nd character are returned) would result in a returned value of "92".
Use the Day, Month or Year functions to extract individual parts of a date.
If you are not using the dates in other calculations, which rely on them being stored as numbers, you can convert the cells containing dates to text, using Excel's Text To Columns tool. To do this:
The data in your selected cells should now be stored as text within Excel, and the Mid function will work as expected.