Re: os.time() and timezones

[Rena <hyperhacker@...>]

On Fri, Apr 13, 2012 at 14:26, Sean Conner <sean@conman.org> wrote:
> It was thus said that the Great steve donovan once stated:
>> On Fri, Apr 13, 2012 at 10:16 AM, Rena <hyperhacker@gmail.com> wrote:
>> > Ouch.) Does Lua provide a way to turn a table into a Unix timestamp
>> > under the assumption that the table specifies GMT, not the local
>> > timezone?
>
>  Even operating systems have trouble with timezones:
>
> http://www.chronos-st.org/Discovering%20the%20Local%20Time%20Zone--Why%20It%27s%20a%20Hard%20Problem.html
>
>> I use this kind of logic to find the timezone offset in pl.Date:
>>
>>         local t = os.time()
>>         local ut = os.date('!*t',t)
>>         local lt = os.date('*t',t)
>>         thour = lt.hour - ut.hour
>>         tmin = lt.min - ut.min
>>
>> If this is misguided I would certainly like to know!
>
>  It is.  Try this:
>
> gmt = {  0 ,  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 , 10 , 11 ,
>        12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 }
> loc = { 19 , 20 , 21 , 22 , 23 ,  0 ,  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,
>         7 ,  8 ,  9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 }
>
> for i = 1 , 24 do
>  print(i,loc[i] - gmt[i])
> end
>
> And even when it does work, it still doesn't take into account DST.  I found
> this works:
>
> now   = os.time()
> lmt   = os.date("*t",now)
> gmt   = os.date("!*t",now)
> timel = os.time(lmt)
> timeg = os.time(gmt)
> zone  = os.difftime(timel,timeg)
>
> if lmt.isdst then
>  if zone < 0 then
>    zone = zone + 3600
>  else
>    zone = zone - 3600
>  end
> end
>
>  -spc (Time is hard!  I wonder how The Doctor does it?)
>
>

Hmm, are you sure about that DST correction? Perhaps I'm not parsing
HTTP dates properly:
function rena_http.parse_date(date)
	local month_names = { jan=1, feb=2, mar=3, apr=4,  may=5, jun=6, jul=7,
		aug=8, sep=9, oct=10, nov=11, dec=12}
	local day, mon, year, hour, min, sec = date:match(
		'...,? (%d-) (...) (%d-) (%d-):(%d-):(%d-)')

	month = month_names[mon and mon:lower()]
	if not (year and month and day and hour and min and sec) then return nil end
	local ok, res = pcall(os.time,
		{year=year, month=month, day=day, hour=hour, min=min, sec=sec})
	return ok and res or nil
end

I send a last-modified date of: Wed, 12 Jan 2011 08:22:15 GMT
which I get from os.date('!%a, %d %b %Y %H:%M:%S GMT', 1294820535)
Firefox subsequently sends the same date in an If-Modified-Since
header, but the above code parses it to a timestamp 7 hours behind the
current time, which I assume must be because of os.time() assuming the
table is local time. The local timezone is actually -0600 but I guess
DST accounts for the extra hour. But Sean's code only gives the
correct offset if I remove the "if lmt.isdst" check...

(man, whose idea was it to use local, human-readable dates for HTTP? I
at least send out X-Unix-Time and X-Lastmodified-Unix headers, just a
shame the browsers don't do the same...)

-- 
Sent from my toaster.