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

[Luca de Alfaro]

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!

This is a basic operation, and it is a big let down that there is no such
library function in Ocaml.
I will have to write it from basic principles.
Anybody has a better suggestion?

Many thanks,

Luca

# let tmrec = {tm_sec = 38; tm_min = 57; tm_hour = 3; tm_mday = 11; tm_mon =
2; tm_year = 107; tm_wday = 0; tm_yday = 0; tm_isdst = false};;
# Unix.mktime tmrec;;
- : float * Unix.tm =
(1173610658.,
{tm_sec = 38; tm_min = 57; tm_hour = 3; tm_mday = 11; tm_mon = 2; tm_year
= 107; tm_wday = 0; tm_yday = 69; tm_isdst = true})

# let tmrec = {tm_sec = 38; tm_min = 57; tm_hour = 2; tm_mday = 11; tm_mon =
2; tm_year = 107; tm_wday = 0; tm_yday = 0; tm_isdst = false};;
# Unix.mktime tmrec;;
- : float * Unix.tm =
(1173610658.,
{tm_sec = 38; tm_min = 57; tm_hour = 3; tm_mday = 11; tm_mon = 2;
tm_year = 107; tm_wday = 0; tm_yday = 69; tm_isdst = true})

[blue storm]

If you want a fine-grained timezones and daylight saving times handling, you
may use a dedicated library for date and time, such that Calendar (wich is
used by PG'OCaml to represent SQL date/time values) :
http://calendar.forge.ocamlcore.org/

[Erik de Castro Lopo]

blue storm wrote:

I really like the way the Netdate module handles times and dates.

Erik
--
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/

_______________________________________________
Caml-list mailing list. Subscription management:
http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
Archives: http://caml.inria.fr
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
Bug reports: http://caml.inria.fr/bin/caml-bugs

[Yaron Minsky]

The TZ module in Core could also be helpful here.

y

[Dave Benjamin]

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

[Luca de Alfaro]

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

[Luca de Alfaro]

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