If you look at the below screenshot, my example of counting timestamps, you'll see that if I'm bringing in NULL values Tableau will display them if 'Show Missing Values' at the lowest level of granularity (hour in this case) is checked.
Because there is no data, my initial result was the hour would display with simply blank space. So there'd be a value for 1am and 2am but for 3am it was simply blank. To show '0', wrap your calculation with ZN like this: ZN(COUNT([Timestamp])) to force Tableau to display a '0'. Hope this helps! Thx, Don