I'm forwarding this message from John Dlugosz, who is not on the time zone mailing list. Those of you who are on the list, please direct replies appropriately. --ado -----Original Message----- From: John Dlugosz [mailto:JDlugosz@TradeStation.com] Sent: Tuesday, January 06, 2009 7:10 To: tz@lecserver.nci.nih.gov Subject: time zone library I downloaded the timezone library, and in adapting it to my needs I've been figuring out how it works. I have a few questions, and also (hopefully) a contribution. First, my contribution. Regarding /* ** Adapted from code provided by Robert Elz, who writes: ** The "best" way to do mktime I think is based on an idea of Bob ** Kridle's (so its said...) from a long time ago. ** It does a binary search of the time_t space. Since time_t's are ** just 32 bits, its a max of 32 iterations (even at 64 bits it ** would still be very reasonable). */ I'm using 64-bit time_t values, and find that it usually takes 62 to 64 iterations. Basically, it is always worst case. I tried using a rough guess for the initial hi and low: lo = ((yourtm.tm_year-70) * 365i64 + yourtm.tm_mon * 28) * SecondsPerDay; hi = ((yourtm.tm_year-70) * 366i64 + (yourtm.tm_mon+1)*31) * SecondsPerDay; when the resulting time_t would be positive, and it completes in 20 to 23 iterations. I punted on the negative case, but a very similar calculation would work. When tm_year < 70, I just used the smallest value I care about for lo, and 0 for hi. That gives me worst case 35 iterations, because I'm only supporting values back to 1752 CE, not the full range of a 64-bit number, which would be 21 times longer than the age of the universe. Now for my first question. TZ_MAX_TIMES is 1200, and that is exactly the length of the arrays for state::ats and state::types. I suppose it is noting every time change, e.g. DST adjustments twice a year for 600 years? There is no need to store anything before the first reported time in the configuration files, as it simply uses the first entry for anything older (no DST in pre-history); it seems to have code for dealing with dates beyond the end of the table. Does it extrapolate the current DST rule in the future? I'd like to cut off the table at a more reasonable date for my application. Where is that table populated? Is it when the data file is loaded, or in zic when it is created? My second question concerns leap seconds. It appears that the leap second data is associated with every zone file. Why isn't it simply the same for everything, and global? --John