A serial number is a unique, identifying number or group of numbers and letters assigned to an individual piece of hardware or software. Other things have serial numbers as well, though, including banknotes and other similar documents.
The idea behind serial numbers is to identify a specific item, much like how a fingerprint identifies a specific person. Instead of some names or numbers that specify a whole range of products, a serial number is intended to provide a unique number to one device at a time.
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Hardware serial numbers are embedded in the device, while software or virtual serial numbers are sometimes applied to the user who will be using the software. In other words, a serial number used for software programs are tied to the purchaser, not that specific copy of the program.
The term serial number is often shortened to just S/N or SN, especially when the word precedes an actual serial number on something. Serial numbers are also sometimes, but not often, referred to as serial codes.
Serial Numbers Are Unique
It's important to distinguish serial numbers from other identifying codes or numbers. In short, serial numbers are unique.
For example, a model number for a router might be EA2700 but that's true for every single Linksys EA2700 router; the model numbers are identical while each of their serial numbers is unique to each particular component.
As an example, if Linksys sold 100 EA2700 routers in one day from their website, every one of those devices would have 'EA2700' somewhere on them and they would look identical to the naked eye. However, each device, when first built, had serial numbers printed on most of the components that are not the same as the others bought that day (or any day).
UPC Codes are common as well but are actually not unique like serial numbers. UPC Codes are different than serial numbers because UPC Codes are not unique to each individual piece of hardware or software, as serial numbers are.
The ISSN used for magazines and ISBN for books is different as well because they're used for whole issues or periodicals and aren't unique for every instance of the copy.
Hardware Serial Numbers
You've probably seen serial numbers many times before. Nearly every piece of the computer has a serial number including your monitor, keyboard, mouse and sometimes even your entire computer system as a whole. Internal computer components like hard drives, optical drives, and motherboards also feature serial numbers.
Serial numbers are used by hardware manufacturers to track individual items, usually for quality control.
For example, if a piece of hardware is recalled for some reason, customers are usually made aware of which particular devices need service by being provided a range of serial numbers.
Serial numbers are also used in non-tech environments like when keeping an inventory of tools borrowed in a lab or shop floor. It's easy to identify which devices need to be returned or which ones have been misplaced because each of them can be identified by their unique serial number.
Software Serial Numbers
Serial numbers for software programs are usually used to help ensure that the program's installation is only performed one time and only on the purchaser's computer. Once the serial number is used and registered with the manufacturer, any future attempt to use that same serial number can raise a red flag since no two serial numbers (from the same software) are alike.
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If you're planning on reinstalling a software program you've purchased, you'll sometimes need the serial number to do so. See our guide on how to find a serial key if you need to reinstall some software.
Sometimes, you might find that a software program can attempt to make a serial number for you that you can use to activate a program illegally (since the code wasn't legally purchased). These programs are called keygens (key generators) and should be avoided.
A serial number for a piece of software is not usually the same as a product key but they are sometimes used interchangeably.
The serial number, or serial date, is the number Excel uses to calculate dates and times entered into a worksheet. The serial number is calculated either manually or as a result of formulas involving date calculations. Excel reads the computer's system clock to keep track of the amount of time that has elapsed since the date system's start date.
Note: The information in this tutorial applies to Excel 2019, Excel 2016, Excel 2013, Excel 2010, Excel 2007, and Excel for Mac.
Two Possible Date Systems
By default, versions of Excel that run on Windows store the date as a value representing the number of full days since midnight January 1, 1900, plus the number of hours, minutes, and seconds for the current day.
Versions of Excel that run on Macintosh computers default to one of the following two date systems:
- Excel for Mac versions 2019, 2016, and 2011: The default date system is the 1900 date system which guarantees date compatibility with Excel for Windows.
- Excel 2008 and older versions: The default date system begins on January 1, 1904 and is referred to as the 1904 date system.
All versions of Excel support both date systems and it's possible to change from one system to the other.
Serial Number Examples
In the 1900 system, the serial number 1 represents January 1, 1900, 12:00:00 a.m. while the number 0 represents the fictitious date January 0, 1900.
In the 1904 system, the serial number 1 represents January 2, 1904, while the number 0 represents January 1, 1904, 12:00:00 a.m.
Times Stored as Decimals
Times in both systems are stored as decimal numbers between 0.0 and 0.99999, where:
- 0.0 is 00:00:00 (hours:minutes:seconds)
- 0.5 is 12:00:00 (12 p.m.)
- 0.99999 is 23:59:59
To show dates and times in the same cell in a worksheet, combine the integer and decimal portions of a number.
For example, in the 1900 system, 12 p.m. on January 1, 2016, is serial number 42370.5 because it is 42370 and one-half days (times are stored as fractions of a full day) after January 1, 1900. Similarly, in the 1904 system, the number 40908.5 represents 12 p.m. on January 1, 2016.
Serial Number Uses
Serial Number Examples
Many projects that use Excel for data storage and calculations use dates and times in some way. For example:
- A long-term project that counts the number of days between current and past dates using the NETWORKDAYS function.
- A loan calculation that determines a future date using the EDATE function.
- Time sheets that calculate the elapsed time between start and end times, as well as hours, and overtime as necessary using formulas that add or subtract dates and times.
- Time stamping a worksheet with the current date and time with keyboard shortcuts that read the current serial number.
- Updating the displayed date and time whenever a worksheet is opened or recalculated with the NOW and TODAY functions.
Only one date system can be used per workbook. If the date system for a workbook that contains dates is changed, those dates shift by four years and one day due to the time difference between the two date systems.
Serial Number Examples
Change the Default Date System
To set the date system for a workbook in Excel running on a Windows PC:
- Open the workbook to be changed.
- Select File. Except in Excel 2007, where you select the Office button.
- Select Options to open the Excel Options dialog box.
- Select Advancedin the left-hand panel of the dialog box.
- In the When calculating this workbook section, select or clear the Use 1904 date system check box.
- Select OK to close the dialog box and return to the workbook.
To set the date system for a workbook in Excel for Mac:
- Open the workbook to be changed.
- Select the Excelmenu.
- Select Preferences to open the Excel Preferences dialog box.
- In the Formulas and List section, select Calculation.
- In the When Calculating Workbooks section, select or clear the Use 1904 date system check box.
Why Two Date Systems?
PC versions of Excel (Windows and DOS operating systems) initially used the 1900 date system for the sake of compatibility with Lotus 1-2-3, the most popular spreadsheet program at the time.
The problem is that when Lotus 1-2-3was created, the year 1900 was programmed in as a leap year when in fact it was not. As a result, additional programming steps were needed to correct the error. Current versions of Excel keep the 1900 date system for the sake of compatibility with worksheets created in previous versions of the program.
Since there was no Macintosh version of Lotus 1-2-3, initial versions of Excel for Macintosh did not need to be concerned with compatibility issues. The 1904 date system was chosen to avoid the programming problems related to the 1900 non-leap-year issue.
On the other hand, it did create a compatibility issue between worksheets created in Excel for Windowsand Excel for the Mac. This is why all new versions of Excel use the 1900 date system.