2.1. Adding Different Types of Data
One of Excel's most important features is its ability to distinguish between different types of information. A typical worksheet contains both text and numbers. There isn't a lot that you can do in Excel with ordinary text (other than alphabetize a list, perform a simple spell check, and apply some basic formatting). On the other hand, Excel gives you a wide range of options for numeric data. For example, you can string your numbers together into complex calculations and formulas, or you can graph them on a chart. Programs that don't try to separate text and numberslike Microsoft Word, for examplecan't provide these features.
Most of the time, when you enter information in Excel, you don't explicitly indicate the type of data. Instead, Excel examines the information you've typed in, and, based on your formatting and other clues, classifies it automatically. Excel distinguishes between four core data types:
Ordinary text . This data type includes column headings, descriptions, and any content that Excel can't identify as one of the other data types.
Numbers . This data type includes prices, integers, fractions, percentages, and every other type of numeric data. Numbers are the basic ingredient of most Excel worksheets.
Dates and times . This data type includes dates (like Oct 3, 2004), times (like 4:30 PM), and combined date and time information (like Oct 3, 2004, 4:30 PM). You can enter date and time information in a variety of formats.
True or false values . This data type (known in geekdom as a Boolean value) can contain one of two things: TRUE or FALSE (displayed in all capitals). You don't need Boolean data types in most worksheets, but they're useful in worksheets that include Visual Basic macro code (see Chapter 25) or that use complex formulas that evaluate conditions (see Chapter 12).
One useful way to tell how Excel is interpreting your data is to look at cell alignment, as explained in Figure 21.

As Figure 21 shows, Excel can display numbers and dates in several different ways. For example, some of the numbers include decimal places, one uses a comma, and one has a currency symbol. Similarly, one of the time values uses the 12hour clock while another uses the 24 hour clock, and other entries include only date information or both date and time information. You might assume that when you type in a number, it appears in the cell exactly the way you typed it. For example, if you type 3comma000 you expect to see 3,000. However, this isn't always what happens. To see the problem in action, try typing 3,000 in a cell. It shows up exactly the way you entered it. Then, type over that value with 2000 . The new number appears as 2,000. Excel remembers your first entry, and assumes that you want to use thousand separators in this cell all the time .
These differences may seem like a spreadsheet freeforall, but don't despairyou can easily set the formatting of numbers and dates. (In fact, that's the subject of Chapter 4.) At this point, all you need to know is that the values Excel stores in each cell don't need to match exactly the values that it displays in each cell. For example, the number 4300 could be formatted as plain old 4300 or as the dollar amount $4,300. Excel lets you format your numbers so you have exactly the representation you want. At the same time, Excel treats all numbers equivalently, no matter how they're formatted, which lets you combine them together in calculations. Figure 22 shows you how to find the underlying stored value of a cell.

By looking at cell alignment, you can easily tell how Excel is interpreting your data. That's helpful. But what happens when Excel's interpretation is at odds with your wishes? For example, what if you type in something you consider a number but Excel freakishly treats it as text , or vice versa? The first step to solving this problem is grasping the logic behind Excel's automatic decisionmaking process.
If your cell meets any of the following criteria, Excel automatically treats the content as ordinary text :
It contains any letters . Thus, C123 is text, not a number.
It contains any punctuation that Excel can't interpret numerically . Punctuation allowed in numbers and dates includes the comma (,), the decimal point (.), and the forward slash (/) or dash () for dates. If you type in any other punctuation, Excel treats the cell as text. Thus, 14! is text, not a number.
Occasionally, Excel reads your data the wrong way. For example, you might have a valuelike a social security number or a credit card numberthat's made up entirely of numeric characters but that you want to treat like text because you don't ever want to perform calculations with it. But Excel doesn't know what you're up to, and so it automatically treats the value as a number. You can also run into problems when you precede text with the equal sign (which tells Excel that you have a formula in progress), or when you use a series of numbers and dashes that you don't intend to be part of a date (for example, you want to enter 123 but you don't want Excel to read it as January 2, 2003which is what it wants to do).
In all these cases, the solution is simple. Before you type the cell value, start by typing an apostrophe ('). The apostrophe tells Excel to treat the cell content as text. Figure 23 shows you how it works.

When you precede a numeric value with an apostrophe, Excel checks out the cell to see what's going on. If Excel determines that it can represent the content as a number, it places a green triangle in the top left corner of the cell and gives you a few options for dealing with the cell, as shown in Figure 24.

Excel automatically interprets any cell that only contains numeric characters as a number. In addition, you can add the following nonnumeric characters to a number without causing a problem:
One decimal point (but not two). For example, 42.1 is a number, but 42.1.1 is text.
One or more commas, provided you use them to separate groups of three numbers (like thousands, millions, and so on). Thus 1,200,200 is a valid number, but 1,200,20 is text.
A currency sign ($ for U.S. dollars), provided it is at the beginning of the number.
A percent symbol at the beginning or end of the number (but not both).
A plus (+) or minus () sign before the number. You can also create a negative number by putting it in parentheses. In other words, entering (33) is the same as entering 33.
An equal sign at the start of the cell.
The most important thing to understand about entering numbers is that when you choose to add other details like commas or the dollar sign, you're actually doing two things at once: you're entering a value for the cell and you're setting the format for the cell, which affects how Excel displays the cell. Chapter 4 provides more information about number styles and shows how you can completely control cell formatting.
When typing in a date, you have a choice of formats. You can type in a full date (like July 4, 2004 ) or you can type in an abbreviated date using dashes or slashes (like 742004 or 7/4/2004 ), which is generally easier. If you enter some numbers formatted as a date, but the date you entered doesn't exist (like the 30 ^{ th } day in February or the 13th month), Excel interprets it as text. Figure 25 shows you the options.

Because you can represent dates a few different ways, working with them can be tricky, and you're likely to encounter some unexpected behavior from Excel. Here are some tips for using dates, troublefree:
Instead of using a number for the month, you can use a threeletter month abbreviation, but you must put the month in the middle. In other words, you can use 7/4/2004 and 4/Jul/2004 interchangeably.
When you use a twodigit year as part of a date, Excel tries to guess whether the first two digits of the year should be 20 or 19. If the twodigit year is from 00 to 29, Excel assumes it belongs to the 21 ^{ st } century. If the year is from 30 to 99, Excel plants it in the 1900s. In other words, Excel translates 7/4/29 into 7/4/2029, while 7/4/30 becomes 7/4/1930.
If you don't type in any year at all, Excel automatically assumes you mean the current year . For example, if you enter 7/4, Excel inserts the date 7/4/2005 ( assuming it's currently 2005 on your computer's internal clock). When you enter a date this way, the year component doesn't show up in the cell, but it's still stored in the worksheet (and visible on the Formula bar).
Excel understands and displays dates differently depending on the regional settings on your computer . Windows has a setting that determines how your computer interprets dates (see the box on Sidebar 2.1). On the U.S. system, MonthDayYear is the standard progression. But on the UK system, DayMonthYear is the deal. For example, in the U.S., either 11704 or 11/7/04 is shorthand for November 7, 2004. In the UK, the same notations refer to July 11, 2004.
Thus, if your computer has U.S. regional settings turned on, and you type in 11/7/04 , Excel understands it as November 7, 2004, and the Formula bar displays 11/7/04.
Typing in times is more straightforward than typing in dates. You simply use numbers, separated by a colon (:). You need to include an hour and minute component at minimum (as in 7:30), but you can also add seconds, milliseconds , and more (as in 7:30:10:10). You can use values from 1 to 24 for the hour part, though if your system is set to use a 12hour clock, Excel converts the time accordingly (in other words, 19:30 becomes 7:30 PM.) If you want to use the 12hour clock when you type in a time, just follow your time with a space and then letters P or PM (or A or AM).
Finally, you can create cells that have both date and time information. To do so, just type the date portion first, followed by a space, and then the time portion. For example, Excel happily accepts this combo: 7/4/2004 1:30 PM.
Behind the scenes, Excel stores dates as numbers. It considers the date January 1, 1900 to be day 1. January 2, 1900 is day 2, and so on, up through the year 9999. This system is quite nifty, because if you use Excel to subtract one date from another, you actually end up calculating the difference in days, which is exactly what you want. On the other hand, it means you can't enter a date in Excel that's earlier than January 1, 1900 (if you do, Excel treats your date like text).
UP TO SPEED Regional Dating 
Windows has regional settings for your computer, which affect the way Microsoft programs understand things like dates and currency. You can change the settings, and they don't have to correspond to where you liveyou can set them for your company headquarters on another continent , for instance. But keep in mind that these affect all the programs on your computer. To make a switch, go to the Start menu in Windows and choose Settings Control Panel, then doubleclick Regional and Language Options, which brings up a dialog box. The Regional Options tab has the settings you want. The most important setting is in the first box, which has a dropdown list you can use to pick the region you want, like English (United States) or Swedish (Finland). You can finetune the settings in your region, too. This makes sense only if you have particular preferences about how dates should be formatted that don't match the standard options. Click the Customize button next to the region box to bring up a new dialog box, and then click the Date tab (shown here). Use the pulldown menus to specify the date separator, order of month, day, and year components in a date, and how Excel should interpret twodigit years . You can mix and match these settings freely , although you might wind up with a computer that's completely counterintuitive to other people.
No matter what the regional settings are, you can always use the international date standard, which is Year/Month/Day, though you must supply a fourdigit year (as in 2004/7/4). If you use a twodigit year, Excel will assume you are trying to use the MonthDayYear or DayMonthYear pattern. 
Similarly, Excel stores times as fractional numbers from 0 to 1. The number 0 represents 12:00 AM (the start of the day) and 0.999 represents 11:59:59 PM (the end of the day). As with dates, this system allows you to subtract one time value from another. See Chapter 10 for more information on dates and times.