Excel 2011 for Mac has fantastic date and time calculation capabilities. When using Excel worksheets, you need to know just two secrets: Each day is represented by a whole number, called a serial number, and portions of days are represented by decimal fractions.

Finding today with Excel formulas

Some days, you wake up and don’t even know what day it is. Excel doesn’t have this problem. To have Excel return the current date, select the cell you want Excel to show the current date in and type the cell formula =TODAY() and then click the green Enter button. The selected cell displays today’s date, and Excel automatically changes the format of the cell to Date.

Getting today’s serial number in Excel 2011 for Mac

Each day has its own serial number in Excel. If you follow the steps in the preceding section, you don't see the serial number in the selected cell because Excel knows the formula represents a date. If you want to see the serial number instead of a date format, you can manually change the format of the selected cell to Number by selecting Number in the Ribbon's Number Format pop-up menu.

In Excel, to add and subtract any number of days, just add and subtract whole numbers. You're probably wondering how Excel arrived at the serial number of today. To find out, enter 1 in a cell and apply the Date format. The date changes to January 1, 1900 — the first day that Excel knows about. Every day in Excel is the number of days after 1/1/1900.

Finding the time of day with Excel’s formatting tools

Because Excel works with days as whole numbers, you might guess that portions of days are fractions. Well, you’d be right! Starting with a whole number representing a date, append .5 (one-half day) to a date serial number to represent noon. Apply the Time number format, and the time changes to 12:00 PM. Go ahead and try some different decimals.

Formatting a date in Excel 2011 for Mac

You have at least three different ways to apply a date format. Perhaps the fastest is to select a cell or cell range, and then click the Home tab of the Ribbon. In the Number group, click the pop-up button under the Number group title and choose Date to display the date as m/d/yy, where m represents the month's number, d represents the day number, and yy represents a two-digit year.

Excel has many more built-in date formats, which you can apply by displaying the Format Cells dialog by pressing Command-1 and then clicking the Number tab. You can also display the Number tab of the Format Cells dialog by clicking the Home tab on the Ribbon. Then click the pop-up button under the Number group title and choose Custom from the pop-up menu.

When the Format Cells dialog displays, select the Date category. Choose a Type from the list. Choosing a different Location (language) or Calendar type changes the date types offered.