Convert substitute() function to lua

I have a function to change the file timestamp, I was been using the :s for substitution but the substitute() function does not mess with jumps and search patterns, making it better swited for the task, so until now I have the following:

--> :lua changeheader()
-- This function is called with the BufWritePre event (autocmd)
-- and when I want to save a file I use ":update" which
-- only writes a buffer if it was modified
M.changeheader = function()
    -- We only can run this function if the file is modifiable
    local bufnr = vim.api.nvim_get_current_buf()
    if not vim.api.nvim_buf_get_option(bufnr, "modifiable") then
        require("notify")("Current file not modifiable!")
    -- if not vim.api.nvim_buf_get_option(bufnr, "modified") then
    --     require("notify")("Current file has not changed!")
    --     return
    -- end
    if vim.fn.line("$") >= 7 then
    os.setlocale("en_US.UTF-8") -- show Sun instead of dom (portuguese)
    local time ="%a, %d %b %Y %H:%M:%S")
    local l = 1
    while l <= 7 do
        vim.fn.setline(l, vim.fn.substitute(vim.fn.getline(l), 'last change: \\zs.*', time , 'gc'))
        l = l + 1

        require("notify")("Changed file header!")

This function is defined in a file called utils.lua that has at the beginning an empty table where we define the local modules:

local M = {}

At the end of the same file we have:

return M

This way we can call the function in this fashion:

lua require("core.utils").changeheader()<CR>

It seems like the last update has solved the issue, please test this function and give us some feedback and ideas!