The Excel Left function returns a specified number of characters from the start of a supplied text string.
The syntax of the function is:
Where the function arguments are:
text    The original text string. 
[num_chars]   
An optional argument that specifies the number of characters to be returned from the start of the supplied text. If omitted, the [num_chars] argument takes on the default value of 1. 
Note that the Left function always returns a text string, even though this may be contain digits and may look like a number. This may be important if you wish to use the result of the function within further functions or formulas.
Column B of the following spreadsheet shows three examples of the Excel Left function.
Formulas:

Results:

Note that in the above examples:
For further information and examples of the Excel Left function, see the Microsoft Office website.
If you get an error from the Excel Left function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied [num_chars] argument is < 0. 
Also, some users encounter the following problem:
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 start, mid or end characters of the number that represents that date.
For example, 01/01/1980 is represented by the number 29221, so applying the Left function to a cell containing the date 01/01/1980 (and requesting that 4 characters be returned) would result in a returned value of "2922".
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 Left function will work as expected.