I have a similar setup to GP: Emacs + one file + an "new-day" command.

Everything goes in one big "scratch.txt" file, with headers like:

    ###### Saturday, May 20 2017

...at the start of each day. These are generated by a function I run every day:

    (defun new-day ()
      (interactive)
      (insert (format-time-string "###### %A, %B %d %Y\n")))

"scratch.txt" is always an open buffer in Emacs. You can search through the file with isearch, helm, or whatever you use. I use the "desktop" library to save and restore all open files when I close Emacs.

That's really it, just one big unstructured text file.

I don't put a lot of prose in this diary. Only todo items, code snippets, and errors / log output.

Our setups have slightly different goals: yours is to create daily nuggets of documentation, while mine is to archive whatever thoughts and code were running through my head each day.