mjhjmtu

You can use the expression register, "=, with p (or P) in normal mode or <C-R> in insert mode:

In normal mode:
_{(<C-M> here means Control+M, or just press Enter/Return)}

"=strftime('%c')<C-M>p

In insert mode:
_{(<C-M> has the same meaning as above, <C-R> means Control+R)}

<C-R>=strftime('%c')<C-M>

If you want to insert the result of the same expression many times, then you might want to map them onto keys in your .vimrc:
_{(here the <C-M> and <C-R> should be typed literally (a sequence of five printable characters—Vim will translate them internally))}

:nmap <F2> "=strftime('%c')<C-M>p
:imap <F2> <C-R>=strftime('%c')<C-M>

:r!date +%c

see :help :r!

These commands will insert the output of strftime("%c") right where your cursor is:

:exe ":normal i" . strftime("%c")

and

:call feedkeys("i". strftime("%c"))

There are other ways to do what you want (like, for example, those on Mikel's answer).

Edit: Even better, for in-place insert, use the = register as Chris Johnsen describes

If you want to insert the output of a vim command (as opposed to the return value of a function call), you have to capture it. This is accomplished via the :redir command, which allows you to redirect vim's equivalent of standard output into a variable, file, register, or other target.

:redir is sort of painfully inconvenient to use; I would write a function to encapsulate its functionality in a more convenient way, something like

funct! Exec(command)
 redir =>output
 silent exec a:command
 redir END
 return output
endfunct!

Once you've declared such a function, you can use the expression register (as explained by Chris Johnsen) to insert the output of a command at the cursor position. So, from normal mode, hit i^R=Exec('ls') to insert the list of vim's current buffers.

Be aware that the command will execute in the function namespace, so if you use a global variable you will have to explicitly namespace it by prefixing it with g:. Also note that Exec(), as written above, will append a terminating newline to even one-line output. You might want to add a call to substitute() into the function to avoid this.

Also see https://stackoverflow.com/questions/2573021/vim-how-to-redirect-ex-command-output-into-current-buffer-or-file/2573054#2573054 for more blathering on about redir and a link to a related command.

:call append(line('.'), strftime("%c"))

Will put it on the next line, then you could press J (Shift+J)to join it up to the current position.

Or if you need it all in one command, you could do

:call setline(line('.'), getline(line('.')) . strftime("%c"))

or

:call setline(line('.'), getline(line('.')) . " " . strftime("%c"))

depending on whether you want a space inserted before the date or not.

Improving @intuited answer to avoid the problem with leading whitespace and growing indent:

"Examples:
":call Exec('buffers')
"This will include the output of :buffers into the current buffer.
"
"Also try:
":call Exec('ls')
":call Exec('autocmd')
"
funct! Exec(command)
 redir =>output
 silent exec a:command
 redir END
 let @o = output
 execute "put o"
 return ''
endfunct!

This will simply insert at the current location in the file when you :call Exec('command') from normal mode. As noted in the comment, the original (insert-mode) Ctrl+R =Exec('command') approach with Exec(..) returning a string could be partially corrected by using set paste, but doesn't offer an opportunity to put the set nopaste anywhere.

The let @o = output syntax sets the register o to the contents of the variable output, as explained here: https://stackoverflow.com/a/22738310/1143274

The return '' line is so that the default return value of 0 doesn't get inserted to the buffer.