Richtextbox.text into .txt file, choose path/filename with DateTimePicker Selection?

I want to save text from a Richtextbox into a .txt file using this code.
But I don't know how to accurately choose destination path
and I would like to make the file name the day, date, and year that you can pick from using
the "DateTimePicker" component.

(My example in code)
that way when they choose a specific date from the "DateTimePicker" I can write a command to open a .txt file from that date (which will be named that date)

Thank you.