Excel's Volatile NOW Function for the Date and Time

One of Microsoft Excel's best-known date functions is the NOW function. The NOW function adds the current date or time to a worksheet and can also calculate a value based on the current date and time, and have that value updated every time you open the worksheet.

How to Enter the NOW Function

Like most Excel functions, the NOW function is entered into a worksheet using the function's dialog box. Because it takes no arguments, the function can be entered into the active cell by typing =Now() and pressing Enter. The result displays the current date and time.

To change the information displayed, adjust the cell's formatting to show just the date or the time using the Format tab.

Shortcut Keys to Formatting Date and Time

Keyboard shortcuts help you quickly format the NOW function output. For the date (date-month-year format), enter Ctrl+Shift+#. For the time (hour-minute-second and a.m./p.m. format), enter Ctrl+Shift+@.

Serial Number or Date

The reason the NOW function takes no arguments is that the function obtains its data by reading the computer's system clock. Windows versions of Excel store the date as a number representing the number of full days since midnight January 1, 1900, plus the number of hours, minutes, and seconds for the current day. This number is called a serial number or serial date.

Since the serial number continually increases with each passing second, entering the current date or time with the NOW function means the function's output continually changes.

Volatile Functions

The NOW function is a member of Excel's group of volatile functions, which recalculate or update every time the worksheet in which they're located recalculates, just as SUM and OFFSET do.

For example, worksheets recalculate each time they're opened or when certain events occur, such as when you enter or change data, so the date or time changes unless automatic recalculation is turned off.