How to insert the current date into a file using abbrev or similar?

Question

I'd like to replicate something in emacs that I do in AutoHotkey all the time:

1. type td
2. have that string be replaced with the current date

I'm trying to do this with Emacs abbrevs but I'm running into an Invalid function: error.

I've also tried using a builtin function, but I get the same Invalid function error. I've done a bunch of research but no avail.

Here's the code I have currently in my .emacs:

(defun insert-current-iso-date ()
    "..."
    (format-time-string "%Y-%m-%d"))

(define-abbrev-table 'global-abbrev-table
  '(("td" "" (insert-current-iso-date))))

Answer 1

You need to remove the parenthesis from your custom function. It's calling for hook function name there, not its evaluation.

(define-abbrev-table 'global-abbrev-table
  '(("td" "" insert-current-iso-date)))

Anyway, to my understanding, you should define your abbrevs the way it's intended which is using define-abbrev, whose syntax is easier to follow, and not overriding the whole table definition unless you have a good reason to do so.

(define-abbrev global-abbrev-table "td" "" 'insert-current-iso-date)

And, as @NickD pointed:

you have to change the insert-current-iso-date to insert the string: it is not enough to return it as it does currently:

(defun insert-current-iso-date ()
  (insert (format-time-string "%Y-%m-%d"))

And of course you have to turn on abbrev-mode.