DST implementation

[Gnyblast]

How hard is it do you think to implement DST to the library? Because I'm using this lib and since last week I get -1 hr difference between my local time and my app.

[frmdstryr]

It should be a matter of adjusting the timezone offset for relevant timezones. Determining when to do that is the part I'm not sure about.

Edit: I think I'm over simplifying..

If you have any ideas feel free to submit a PR.

[frmdstryr]

Here's the DST schedules https://en.m.wikipedia.org/wiki/Daylight_saving_time_by_country

[Gnyblast]

I though of a different implementaion than yours but couldn't find time to implement it.

create method was gonna get a boolean and a enum. boolean for if dst exist for the tz and enum was for defining the start-end for dst.

a factory method to pass enum and get the dst start/end dates, and on Timezone constructor, if dst is true and now is in the range or dst start/end then offset would get calculated accordingly by adding amount of minutes coming from the enum. And since everything else using offset value, there won't be any need for changing all the other methods.

[Gnyblast]

Maybe something like this would be easier or give you some more idea:

pub const Timezone = struct {
    offset: i16, // In minutes
    name: []const u8,

    // Auto register timezones
    pub fn create(name: []const u8, offset: i16, dst: bool, dstSettting: @Type(.enum_literal)) Timezone {
        dst_data = getDstZoneData(dstSettting);//TODO get start, end and shift from a function

        const dstOffset = getDSTOffset(offset, dst, dst_data[0], dst_data[1], dst_data[2]);
        const self = Timezone{ .offset = dstOffset, .name = name};
        return self;
    }

    pub fn offsetSeconds(self: Timezone) i32 {
        return @as(i32, self.offset) * time.s_per_min;
    }

    fn getDSTOffset(offset: i16, dst: bool, dst_start: u16, dst_end: u16, shift: u8) i16 {
        if (!dst) return offset;

        const now_in_seconds = std.time.timestamp();
        if (now_in_seconds >= dst_start and now_in_seconds < dst_end) return offset + shift;

        return offset;
    }
};

and then in timezones.zig:

pub const Nicosia = create("Asia/Nicosia", 120, true, .europe_tz);
pub const Rangoon = create("Asia/Rangoon", 390, false, .no_dst); //no_dst is a place holder for no enum settings etc.

[Gnyblast]

I also pushed a draft that I end-up writing my own lastWeekdayOfMonth, We also need your method for getting Nth weekday of the month.

[Gnyblast]

There are some weirdest implementations Idk how to deal with them honestly.

Maybe for the Friday before last Sunday in March at 02:00, I'm able to get last sunday of March and then I can do an explicit
(-2*24*3600) to the result. But what about: Second Saturday after Ramadan[b] at 02:00.

[Gnyblast]

Most of them are implemented now except Palestine. They are implemented without shift exact time, all takes effect of the day at 00:00. Later I can look into that to implement the shift time too, must be fairly easy. I'm just trying to finalize it today or tomorrow.

[Gnyblast]

Might be a bit overflown with comment in the PR. According to my experience over the issue, this can only get to work with Timezone beign non-pointer and non-const and I implemented that way otherwise it won't work without an allocator.

[Gnyblast]

@frmdstryr

test "get-eastern-island-dst-data" {
    const dst_data = getDstZoneData(2025, .eastern_island_summer_time);
    try std.testing.expectEqual(1757116800, dst_data[0]);
    try std.testing.expectEqual(1743811200, dst_data[1]);
    try std.testing.expectEqual(60, dst_data[2]);
}

This is fixing the test failure in src/dst_factory.zig line 245. And it's OK to close this I guess. I can also open a PR for the fix or you can just get it pushed to master.