Date: Tue, 11 Nov 1997 15:25:00 -0500 From: Tom Peterson (USG) <tomp@zk3.dec.com> Some ideas for addressing this problem are: 1) Expand the date/time boundaries for time zone files to a specific range. The boundaries should be expanded to an arbitrary range, so that our descendants don't have to worry about this problem. Also, while we're on the subject, I'd prefer a format that didn't have any arbitrary limits. For example, I'd like to remove the arbitrary limit of at most 255 time zone abbreviation characters. Personally, I'd prefer a text file, and if it were well designed, it wouldn't take more room than the current format (it'd probably take less). Text is more portable and easier to debug. The current C9X draft also makes some other provisions for time zones that I haven't had time to digest; these other provisions may also require changes to the tz file format. We've also been kicking around ideas for rule timestamps, to help maintenance (e.g. for DHCP). Perhaps it is finally really time for a tz file format redesign. Of course we should maintain a backwards compatibility option for people who need to use the old file format. 2) Find some way to mimic what the zic compiler does at run-time. This might involve generating some kind of algorithmic data for each time zone which can be used to calculate future transition times, etc on-the-fly. I'm not sure this is possible or practical given the varied complexity of time zone rules. The code already needs to deal with arbitrary POSIX.1 strings at run-time, so it should be practical to do this. One possibility is to compile rules into POSIX.1 strings in the table, and then decode this at run-time. with 64-bit time_t's, the tm struct becomes the limiting factor. Boundary checks now become an issue for localtime()/gmtime() instead of mktime(). This is a problem for standards at a mininum. While mktime() has the (somewhat ambiguous) -1 return value for non-representable times, no such provision is defined for localtime()/gmtime(). The C Standard says that gmtime can return a null pointer, and this certainly seems like the right thing to do if tm_year cannot represent the year. I believe that current practice is for localtime to return a null pointer with an unrepresentable time (e.g. negative time_t values on some hosts), even though the C Standard does not allow for this. This sounds like a bug in the C Standard, and perhaps it can be fixed in C9X.