Get the suffix for the date string
I'm concerned that make-suffix is sufficiently general that we'll have name collisions with other packages - or even new Emacs versions.  Perhaps prefix with some tag (I tend to begin names with my initials to disambiguate my own functions, for example; that would give you something like an/make-suffix).

Instead of binding a lambda to the keystroke, prefer to give it a name.  That makes it easier to bind to other keys (perhaps interactively) and for Control+h k to give the best output.

Talking of which, let's have some docstrings, please!

Finally, since the function will work only on writable buffers, we should have (interactive "*") to avoid wasting effort when used in a read-only context.