As a part of the Advanced search, there is also the Time Range section, where you can choose:

• Time Range – from to – This is related to the From-To dates that you choose in the calendar. For example, if you choose September 1 to December 31 and the time range from 9AM to 12PM, the results will present all the emails that were sent between September 1 at 9AM to December 31 at 12PM.
  
  
• Time Range – in between  – This is related to the From-To dates that you choose in the calendar. In this case, the time range will affect every date in the chosen range.
  For example, if you choose September 1 to December 31 and the time range from 9AM to 12PM, the results will present all the emails that were sent every day between September 1 to December 31 from 9AM to 12PM. This means that if an email was sent on October 10 at 8AM or 3PM, it will not be presented.

If you don't select anything specific for the time range, the default time range is 00:00 to 23:59, so all the results between the chosen dates will be presented.