The NOW function is useful when you need to display the current date and time on a worksheet or calculate a value based on the current date and time, and have that value updated each time you open the worksheet.
NOW function return the current date and time of system. If the cell format was General before the function was entered, Excel changes the cell format to the same date and time format that is specified in the regional date and time settings in Control Panel. You can change the date and time format for the cell by using the commands in the Number group of the Home tab on the Ribbon.
= NOW ( )