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 <d...@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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