find $HOME -maxdepth 3 -type f -atime -7 \( -name '*.txt' -or -name '*.md' \)

Find .txt and .md files that were accessed within the past week.

The -atime -7 controls the time here. It can also be used to find files accessed more than a week ago (-atime +7), or exactly a week ago -atime 7.

The other interesting part are the parentheses, which are used to group the -name options together, so that -or works only on those two. If the parentheses were not used, -or would have either matched everything before it or everything after it.

-maxdepth 3 is used so that the search space is small enough. With it enabled, the command completes almost instantaneously:

0.07s user 0.06s system 98% cpu 0.139 total

Whereas -maxdepth 4 is already much slower.

0.31s user 0.97s system 37% cpu 3.403 total

This works on my system because my notes are in relatively high-up directories.

Update:

Instead of the command above, I now use one that sorts the files by access time and returns all files it finds, not only the more recent ones:

ls -1t $(find $HOME -maxdepth 2 -type f \( -name '*.txt' -or -name '*.md' \) \! -path "$HOME/.*")

This runs as a cronjob and its output is redirected into $HOME/.recent.txt, where it is then read by Emacs.