" Borrowed from https://gist.github.com/mllg/5353184
" It's like `find ~/.vim/tmp/undo -type f -mtime +31 -delete` but portable
function Tmpwatch(path, days)
    let l:path = expand(a:path)
    if isdirectory(l:path)
        for file in split(globpath(l:path, "*"), "\n")
            if localtime() > getftime(file) + 86400 * a:days && delete(file) != 0
                echoerr "Tmpwatch(): Error deleting '" . file . "'"
            endif
        endfor
    else
        echoerr "Tmpwatch(): Directory '" . l:path . "' not found"
    endif
endfunction

" undofile part of your .vimrc
if exists("+undofile") && !empty(&undodir) && (&undodir != '.')
    " remove undo files which have not been modified for 31 days
    call Tmpwatch(&undodir, 31)
endif