The function NOW() takes no argument and returns the current date and time combined into one number. This number is the sum of the serial date number and the serial time number.
Note that the value of this function is not fixed and returns a different value each time the spreadsheet is recalculated.
NOW() returned 77695.71806 on September 20, 2013 at 17:14 PM.
77695.71806 is the sum of DATEVALUE("20.09.2013") and TIMEVALUE("17:14")