You can perhaps use `strftime` instead of `trim(system('date ..))`:

   inoremap <Leader>date <C-r>=strftime("%a %B %e, %Y %H:%M:%S %p")<CR>
   nnoremap <Leader>date :put=strftime("%a %B %e, %Y %H:%M:%S %p")<CR>

I am also not sure if an `<esc>` is really necessary at the end of your normal mode map.