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 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 > . > > 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 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 >> > >