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

datetime.lua

function parse_json_date(json_date)
    local pattern = "(%d+)%-(%d+)%-(%d+)%a(%d+)%:(%d+)%:([%d%.]+)([Z%+%-])(%d?%d?)%:?(%d?%d?)"
    local year, month, day, hour, minute, 
        seconds, offsetsign, offsethour, offsetmin = json_date:match(pattern)
    local timestamp = os.time{year = year, month = month, 
        day = day, hour = hour, min = minute, sec = seconds}
    local offset = 0
    if offsetsign ~= 'Z' then
      offset = tonumber(offsethour) * 60 + tonumber(offsetmin)
      if xoffset == "-" then offset = offset * -1 end
    end
    
    return timestamp + offset
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')))

A little fixed version: time in UTC (not in local time zone), assume os.time returns seconds

function parse_json_date(json_date)
    local pattern = "(%d+)%-(%d+)%-(%d+)%a(%d+)%:(%d+)%:([%d%.]+)([Z%+%-])(%d?%d?)%:?(%d?%d?)"
    local year, month, day, hour, minute, 
        seconds, offsetsign, offsethour, offsetmin = json_date:match(pattern)
    local timestamp = os.time{year = year, month = month, 
        day = day, hour = hour, min = minute, sec = seconds} - os.time{year=1970, month=1, day=1, hour=0}
    local offset = 0
    if offsetsign ~= 'Z' then
      offset = tonumber(offsethour) * 60 + tonumber(offsetmin)
      if xoffset == "-" then offset = offset * -1 end
    end
    
    return timestamp + offset * 60
end

xoffset used but not defined, maybe it should be:

if offsetsign == "-" then offset = offset * -1 end

I've forgotten to update. There's my final version I currently use below.

local epoch = os.time{year=1970, month=1, day=1, hour=0}
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 = 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

I've forked this and fix an error "time result cannot be represented in this installation", but I can't create pull request for the gist...

I've forgotten to update. There's my final version I currently use below.

local epoch = os.time{year=1970, month=1, day=1, hour=0}
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 = 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