Change the zsh heading to unix.

unix/find-newer-files.md

@@ -0,0 +1,13 @@
+# Find Newer Files
+
+Use the `-newer` flag with the name of a file to find files that have a
+newer modification date than the named file.
+
+For instance,
+
+```
+$ find blog -name '*.md' -newer blog/first-post.md
+```
+
+will find all markdown files in the `blog` directory that have a
+modification date more recent than `blog/first-post.md`.