To make it work cross-platform, just put the following in your vimrc:
nmap <F3> i<C-R>=strftime("%Y-%m-%d %a %I:%M %p")<CR><Esc>
imap <F3> <C-R>=strftime("%Y-%m-%d %a %I:%M %p")<CR>
Now you can just press F3 any time inside Vi/Vim and you’ll get a timestamp like 2016-01-25 Mo 12:44 inserted at the cursor.
For a complete description of the available parameters check the documentation of the C function strftime().