simple vim completefunc just a demo

month.vim

let s:month = {
      \ '1': 'Jan',
      \ '2': 'Feb',
      \ '3': 'Mar',
      \ '4': 'Apr',
      \ '5': 'May',
      \ '6': 'Jun',
      \ '7': 'Jul',
      \ '8': 'Aug',
      \ '9': 'Sep',
      \ '10': 'Oct',
      \ '11': 'Nov',
      \ '12': 'Dec'
      \ }

function! month#complete(findstart, base) abort
  if a:findstart
    return match('<cword>', '[1-9]*$')
  elseif empty(a:base)
    return []
  else
    let l:match = get(s:month, a:base, '')
    return [l:match]
  endif
endfunction