Re: [Caml-list] Ocaml can't convert a GMT time into a float!

[Luca de Alfaro <luca@dealfaro.org>]

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Sorry: wrong link.  The code is here:
http://luca.dealfaro.org/converting-gmt-time-into-unix-time-in-ocaml

On Sun, Mar 7, 2010 at 10:05 AM, Luca de Alfaro <luca@dealfaro.org> wrote:

> Thanks to all!
>
> I see, I am glad there are good alternative libraries!
> For my problem, I preferred to avoid having dependence on one more library
> for only 10 lines of code, so I wrote some code to do the conversion.  It
> works only for dates after 1970, and it is somewhat inelegant; it is here:
> http://wghstrfg.blogspot.com/2010/03/i-hate-daylight-savings-time.html
>
> If you want to know why this bug drove me crazy for a couple of evenings,
> you can read this blog post<http://wghstrfg.blogspot.com/2010/03/i-hate-daylight-savings-time.html>
> .
>
> Many thanks, and I am glad my email helped Dave.
>
> Daylight savings time is a huge headache for anyone working on
> historically-timestamped data.
>
> All the best,
>
> Luca
>
>
> On Sat, Mar 6, 2010 at 7:27 PM, Dave Benjamin <dave@ramenlabs.com> wrote:
>
>> Luca de Alfaro wrote:
>>
>>> I need to convert a time, expressed in yyyy/mm/dd hh:mm:ss form, into a
>>> floating point.
>>> The conversion has to be done in GMT, but the real point is, the
>>> conversion must NOT change due to daylight savings time.
>>>
>>> Ocaml seems to have only one conversion function, however: Unix.mktime,
>>> which takes a time, and makes a float in the local time zone.
>>>
>>> No problem, I thought: I will simply add 3600 if the conversion result
>>> tells me that dst is active (and then convert for the difference between GMT
>>> and winter time).
>>> NO! This does not work!  Look at the two conversions below.  The tmrec
>>> differs by one hour.
>>> However, the two floating point numbers returned are identical, and
>>> tm_isdst is set to true in both cases!
>>>
>>> This means that I have no way of using the standard libraries to convert
>>> a time expressed in yyyy/mm/ss hh:mm:ss to a float!
>>>
>>
>> You are absolutely right, and I unfortunately did not notice this subtlety
>> when I wrote the XmlRpcDateTime module that is part of XmlRpc-Light, so this
>> means there is a bug in XmlRpcDateTime.to_unixfloat_utc on systems with time
>> zones that observe daylight savings. I did not notice the problem because I
>> live in Arizona, one of only two states in the US that do not observe
>> daylight savings!
>>
>> The culprit can be seen here in the C implementation of Unix.mktime:
>>
>> CAMLprim value unix_mktime(value t)
>> {
>>  struct tm tm;
>>  time_t clock;
>>  value res;
>>  value tmval = Val_unit, clkval = Val_unit;
>>
>>  Begin_roots2(tmval, clkval);
>>    tm.tm_sec = Int_val(Field(t, 0));
>>    tm.tm_min = Int_val(Field(t, 1));
>>    tm.tm_hour = Int_val(Field(t, 2));
>>    tm.tm_mday = Int_val(Field(t, 3));
>>    tm.tm_mon = Int_val(Field(t, 4));
>>    tm.tm_year = Int_val(Field(t, 5));
>>    tm.tm_wday = Int_val(Field(t, 6));
>>    tm.tm_yday = Int_val(Field(t, 7));
>>    tm.tm_isdst = -1; /* tm.tm_isdst = Bool_val(Field(t, 8)); */
>>    clock = mktime(&tm);
>>    if (clock == (time_t) -1) unix_error(ERANGE, "mktime", Nothing);
>>    tmval = alloc_tm(&tm);
>>    clkval = copy_double((double) clock);
>>    res = alloc_small(2, 0);
>>    Field(res, 0) = clkval;
>>    Field(res, 1) = tmval;
>>  End_roots ();
>>  return res;
>> }
>>
>> The field tm.tm_isdst is not really a boolean from C's perspective. It can
>> be one of *three* states: positive for DST, zero for non-DST, and negative
>> to query the system timezone database for the value. It looks like at one
>> point Unix.mktime was written to take the value you gave it, but this was
>> commented out and the value was fixed to -1. This is why it uses the time
>> zone's daylight savings correction regardless of what you pass in.
>>
>> Would it be possible to have a new function in the standard library with
>> the commented-out behavior instead? As it stands now I don't see any
>> reasonable way to get a UTC float from a Unix.tm.
>>
>> As far as XmlRpc-Light is concerned, I will probably rewrite this function
>> in terms of Netdate, since Ocamlnet is already one of my dependencies.
>> Apologies to anyone who is affected by this bug (hopefully, no one).
>>
>> Thanks,
>> Dave
>>
>
>

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