Another variation:
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" Map <F10> to save after adding date info
map <F10> 1GO% Last saved:<ESC>:r !date<CR>kJ:w<CR>
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1G
goes to the first line in the file
O (that's an uppercase letter O)
opens a new line above the current one and enters
insert mode
% Last saved:
inserts these characters literally
<ESC>
enters command mode
:r !date<CR>kJ
as before, inserts the output of "date" on a new line
below the current one, then joins the new line with
the current line
:w<CR>
writes the current buffer
