How can I insert current date and time into a file using Emacs?

C-u M-! date

Put in your .emacs file:

;; ====================
;; insert date and time

(defvar current-date-time-format "%a %b %d %H:%M:%S %Z %Y"
  "Format of date to insert with `insert-current-date-time' func
See help of `format-time-string' for possible replacements")

(defvar current-time-format "%a %H:%M:%S"
  "Format of date to insert with `insert-current-time' func.
Note the weekly scope of the command's precision.")

(defun insert-current-date-time ()
  "insert the current date and time into current buffer.
Uses `current-date-time-format' for the formatting the date/time."
       (interactive)
       (insert "==========\n")
;       (insert (let () (comment-start)))
       (insert (format-time-string current-date-time-format (current-time)))
       (insert "\n")
       )

(defun insert-current-time ()
  "insert the current time (1-week scope) into the current buffer."
       (interactive)
       (insert (format-time-string current-time-format (current-time)))
       (insert "\n")
       )

(global-set-key "\C-c\C-d" 'insert-current-date-time)
(global-set-key "\C-c\C-t" 'insert-current-time)

Reference

I've used these short snippets:

(defun now ()
  "Insert string for the current time formatted like '2:34 PM'."
  (interactive)                 ; permit invocation in minibuffer
  (insert (format-time-string "%D %-I:%M %p")))

(defun today ()
  "Insert string for today's date nicely formatted in American style,
e.g. Sunday, September 17, 2000."
  (interactive)                 ; permit invocation in minibuffer
  (insert (format-time-string "%A, %B %e, %Y")))

They originally came from journal.el

You can install yasnippet, which will let you type "time" and the tab key, and does a whole lot more besides. It just calls current-time-string behind the scenes, so you can control the formatting using format-time-string.

Here's a package I wrote a while ago that does what you're asking for.

http://github.com/rmm5t/insert-time.el/tree/master/insert-time.el

(require 'insert-time)
(define-key global-map [(control c)(d)] 'insert-date-time)
(define-key global-map [(control c)(control v)(d)] 'insert-personal-time-stamp)

M-1 M-! date

this causes the shell command you run to be inserted into the buffer you are currently editing rather than a new buffer.

The simplest way without shelling out to 'date' is probably:

(insert (current-time-string))

To insert date:

C-c . RET 

For select date Shift left/right/up/down - RET (enter)