Daylight Saving Time

[christop]

How important would it be to have time zones and automatic Daylight Saving Time adjustments on a calculator (specifically, for Punix on a TI-89/TI-92+)? I'm trying to find out if it's a desirable feature or just feature creep.

At a minimum, the C library would have to support the TZ environment variable, so it would be only a bit of overhead in userspace library code only. The kernel wouldn't even have to be aware of timezones or Daylight Saving Time or any of that garbage. As far as the kernel is concerned, the system clock is UTC, as all time zone/DST conversions are done in userspace. For example, the filesystem would store all timestamps in UTC, and user programs, such as ls or date, would convert the timestamps to local time simply by using standard C time functions like localtime() or ctime().*

Optionally, the C library could also support zoneinfo database files. Each zoneinfo file is pretty small­—usually 1 or 2 kB, but given the limited space of the calculator, the user would have timezone files only for each timezone he/she resides in. For example, I live in Arizona, so I would have a zoneinfo file only for Arizona/Phoenix on my calculator.

The first option (TZ variable only) is simple but isn't always accurate, and the second option (zoneinfo database files) is more complicated but is far more accurate (historical DST changes can be stored in the databases). Either way, the rules would have to be maintained on the calculator as the dates and times for DST change once in awhile (DST is a political issue, so the changes are not predictable).


On a side note, I was also thinking about supporting leap seconds, and that would best be done in the kernel, but without an accurate time source like NTP, it would be pretty pointless. With the inaccurate clock on the calculator, you would have to set the clock probably a few times a year to keep it within a few seconds of the real time anyway. Then again, I'm also considering adding frequency adjustments (along the lines of adjtimex(2) in Linux) to help reduce clock skew.

Any comments/questions/flames/praises?




* For the curious types (like me), in POSIX systems you can actually specify a timezone directly on the command line like so: