nodemcu-firmware/lua_examples/timezone/tz.lua

-- tz -- A simple timezone module for interpreting zone files

local M = {}

local tstart = 0
local tend = 0
local toffset = 0
local thezone = "eastern"

function M.setzone(zone)
  thezone = zone
  return M.exists(thezone)
end

function M.exists(zone)
  return file.exists(zone .. ".zone")
end

function M.getzones()
  local result = {}
  for fn, _ in pairs(file.list()) do
    local _, _, prefix = string.find(fn, "(.*).zone")
    if prefix then
      table.insert(result, prefix)
    end
  end
  return result
end

local function load(t)
  local z = file.open(thezone .. ".zone", "r")

  local hdr = z:read(20)
  local magic = struct.unpack("c4 B", hdr)

  if magic == "TZif" then
      local lens = z:read(24)
      local ttisgmt_count, ttisdstcnt, leapcnt, timecnt, typecnt, charcnt -- luacheck: no unused
            = struct.unpack("> LLLLLL", lens)

      local times = z:read(4 * timecnt)
      local typeindex = z:read(timecnt)
      local ttinfos = z:read(6 * typecnt)

      z:close()

      local offset = 1
      local tt
      for i = 1, timecnt do
        tt = struct.unpack(">l", times, (i - 1) * 4 + 1)
        if t < tt then
          offset = (i - 2)
          tend = tt
          break
        end
        tstart = tt
      end

      local tindex = struct.unpack("B", typeindex, offset + 1)
      toffset = struct.unpack(">l", ttinfos, tindex * 6 + 1)
  else
      tend = 0x7fffffff
      tstart = 0
  end
end


function M.getoffset(t)
  if t < tstart or t >= tend then
    -- Ignore errors
    local ok, msg = pcall(function ()
        load(t)
    end)
    if not ok then
      print (msg)
    end
  end

  return toffset, tstart, tend
end

return M