bugAVR C Runtime Library - Bugs: bug #44327, eu_dst() is broken


bug #44327: eu_dst() is broken

Submitted by:  None
Submitted on:  Fri 20 Feb 2015 09:27:48 PM UTC  
Category:  Library Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Header files
Status:  None Assigned to:  None
Percent Complete:  0% Originator Email:  -email is unavailable-
Open/Closed:  Open Release:  Any
Fixed Release:  None

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Fri 11 Oct 2019 09:21:14 AM UTC, comment #1: 

The current algorithm misfires if the month starts on a Thursday, in which case d is calculated to be 31, when it should have been 24.
Adding the following after line 91 fixes it:

if(d==31) d=24;

it does impact the elegance of the algorithm somewhat, and probably performance, but only on months that start on a thursday ;)

I have attached a python script that runs the algorithm 31*7 times, simulating each day of a month (0-30) being each day of the week (0-6), the script prints out an error if any calculation doesn't match the hard-coded truth table.


(file #47647)

Mark Moore <robotto>
Fri 20 Feb 2015 09:27:48 PM UTC, original submission:  

The function eu_dst(), defined in include/util/eu_dst.h, is intended to
implement the European daylight saving rules. There is an error at the
end of the function, where the comparison between the current day of the
month and the day of the last Sunday is reversed. The error is repeated
four times. This makes the function be almost always wrong in March and
October. There is also an off-by-one error earlier in the function:
whenever the last Sunday is the 25th, the function believes it's the

I noticed those errors while trying to benchmark my own implementation
of eu_dst() against the one provided by avr-libc. Then, rather than
fixing the broken function, I suggest replacing it with my

    int eu_dst(const time_t * timer, int32_t * z)
        uint32_t t = *timer;
        if ((uint8_t)(t >> 24) >= 194) t -= 3029443200U;
        t = (t + 655513200) / 604800 * 28;
        if ((uint16_t)(t % 1461) < 856) return 3600;
        else return 0;

This version is based on the fact that the European DST function is
almost periodic. Actually, it is as close to periodic as possible given
the constraint that the transitions are only allowed on Sundays at 01:00
UTC. Since this constraint is also periodic, this allows for a compact
arithmetic solution.

I tested my implementation for correctness on my PC against the system's
localtime(), for every full hour between the epoch (2000-01-01) and the
end of time (2136-02-07). I also tested it on an Arduino Uno
(ATmega328P) against a precomputed table of all the transition times,
both at the transition and the second right before.

In terms of size, this function compiles 52% smaller than the one
provided by eu_dst.h. If we account for the dependencies, it is 75%
smaller (204 vs. 826 bytes). It is also 3 times faster:
about 1250 — 1280 cycles versus 3300 – 4500.


Edgar Bonet.



(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attach Files:

Attached Files
file #47647:  dstCheck.py added by robotto (1KiB - text/x-python - Run this to test the EU_DST algorithm for every possible case of day-of-month/day-of-week combination)


Depends on the following items: None found

Items that depend on this one: None found


Carbon-Copy List
  • -email is unavailable- added by robotto (Updated the item)
  • -email is unavailable- added by the2masters
  • -email is unavailable- added by None (Submitted the item)



    Follow 2 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2019-10-11 robotto Attached File- => Added dstCheck.py, #47647
    2016-09-21 the2masters Carbon-Copy- => Added stefan@the2masters.de

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