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Ocaml can't convert a GMT time into a float!
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Date: 2010-03-07 (03:27)
From: Dave Benjamin <dave@r...>
Subject: Re: [Caml-list] Ocaml can't convert a GMT time into a float!
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 

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).