**Notice**: this answer does not work anymore as `GoogleClock()` has been replaced by the `NOW()` function.

Another option to consider is `=GoogleClock()`. This function differs somewhat from `=NOW()` in that it has a granularity of one minute (rather than one second), but it is guaranteed to update each and every minute, and be correct (to the nearest whole minute) on opening/refreshing the spreadsheet.

If you use:

`=NOW()+GoogleClock()*0`

you will have a granularity of one second, it will update with every edit, and be no less than one minute off when you open the spreadsheet. But it will also update every minute regardless of editing, which may not be desirable.