I don't think there is a way to do that from the Notifications app. It should also be noted that %time% and %date% refer to those of the "triggering" event, not necessarily the current date and time, though if you don't use the "for X minutes" or "repeat every Y" options, they'd be effectively the same.
You certainly could use a rule to do this. Here's an example that includes only the day of week, but you could use this technique (storing it in a variable) to use it later as part of your ultimate TTS string. In this case, a local variable would work, but if you have a use for this elsewhere, you could even do this in its own rule and use a hub variable instead. Maybe some day, the Notifications app will even let you use hub variables so you can do this rest of the work there, too. 
But until then...
