That formula will only work if you enable the iterative calculation as stated by ChemistB in post#5 and moreover this formula is normally used to populate a cell with static time when an entry is made in another cell. In your case A1 will only be populated with the Time value once you enter something in B1 and once B1 is not blank A1 will show you the time when a value was entered in B1 and it will not change automatically unless you delete the content of B1 and re-enter something again in B1 and this time A1 will show you the latest time value.

Hope this helps.