The problem is that even if we will allow for setting time - it still will not be guaranteed to be sent at the specified time.
The Automailer just runs once a day and puts mails into UNA's mailer queue which then sends them by portions (see the setting "Number of messages to send from mail queue per run" in the Notifications settings of UNA). Which makes sense because sending a large amount of emails at once can become a challenge for the server and theoretically even can mark it as a spamserver by mailservers.
Not to mention that there are timezones, i.e. lets say 3PM server time doesn't necessarily mean 3PM recipient's time.