Note that this is a static date/time and the value will not change as time passes by.
However, if you want to have the current date or time to be automatically updated in your spreadsheet, then you have to use the following formulas. These will display the current date and time whenever you open the spreadsheet or switch to a worksheet or recalculate the formulas.
Function which returns the current date =TODAY()
Function which returns the current date and time =NOW()
Since this is a regular spreadsheet function, you can use this in your formulas. For example, you can find out the difference between the current date and a date in future (like a countdown) or to find out the age of a person, given the date of birth.
Note: If you want to display just the current time, then use the NOW() formula and apply a time format to the cell using the Date/Time icon available on the toolbar. Refer the picture on the right.