lua ISO 8601 datetime parser - https://repl.it/IQuI/5

datetime.lua

function get_timezone()
    local now = os.time()
    return os.difftime(now, os.time(os.date("!*t", now)))
end

local epoch = get_timezone()
function parse_json_date(json_date)
    local year, month, day, hour, minute, seconds, offsetsign, offsethour, offsetmin = json_date:match("(%d+)%-(%d+)%-(%d+)%a(%d+)%:(%d+)%:([%d%.]+)([Z%+%- ])(%d?%d?)%:?(%d?%d?)")
    local timestamp = os.time{year = year, month = month, day = day, hour = hour, min = minute, sec = math.floor(seconds)} + epoch
    local offset = 0
    if offsetsign ~= 'Z' then
        offset = tonumber(offsethour) * 60 + tonumber(offsetmin)
        if offsetsign == "-" then offset = -offset end
    end
    return timestamp - offset * 60
end

print(string.format('%d', parse_json_date('2017-05-25T06:21:43Z')))
print(string.format('%d', parse_json_date('2017-05-25T06:21:43+0830')))
print(string.format('%d', parse_json_date('2017-05-25T06:21:43-0400')))
print(string.format('%d', parse_json_date('2017-05-25T06:21:43.213Z')))

print(string.format('%d', parse_json_date('2021-08-20T16:06:25.36825+08:00')))
print(string.format('%d', parse_json_date('2021-08-20T08:06:25.36825Z')))
print(string.format('%d', 1629446785))