NodaTime ZoneIntervals

Question

I'm attempting to build a logical set of time zone records representing a 5-year period where the stop and start date/time matches to what a person perceives as the time change moment daylight savings is observed. For example, in America/New_York id, I want:

03/13/2016 2:00 am    -   11/06/2016  01:59:59 am  gmtOffset:  -4:00
11/06/2016 2:00 am    -   03/12/2017  01:59:59 am  gmtOffset:  -5:00

I am able to build these records, but I see no other way than to start prior to 1912 when dst went in effect and track the End Instant for all zone intervals. I don't see a way to determine the offset in effect when an Interval record period begins other than the previous Interval record. Is there a way to do this using just a single ZoneInterval record?

Question noted in code:

    private void processTZ()
    {
        IDateTimeZoneProvider provider = DateTimeZoneProviders.Tzdb;

        using (StreamWriter sw = new StreamWriter(@"c:\tzOut.txt"))
        {
            foreach (string id in provider.Ids)
            {
                sw.WriteLine("Id: {0}", id);
                DateTimeZone zone = provider[id];

                // Time period I wish to create logical representation.
                Instant instantValidityBegin = (new LocalDateTime(2014, 1, 1, 0, 0)).InZoneLeniently(zone).ToInstant();
                Instant instantValidityEnd = (new LocalDateTime(2019, 12, 31, 23, 59, 59, 0)).InZoneLeniently(zone).ToInstant();

                // Time period from zone intervals to iterate. The first year dst was observed was 1916.
                Instant instantFetchYearBegin = (new LocalDateTime(1916, 1, 1, 0, 0)).InZoneLeniently(zone).ToInstant();
                Instant instantFetchYearEnd = (new LocalDateTime(2019, 12, 31, 23, 59, 0)).InZoneLeniently(zone).ToInstant();

                // Get the intervals
                IEnumerable<NodaTime.TimeZones.ZoneInterval> intervals = zone.GetZoneIntervals(instantFetchYearBegin, instantFetchYearEnd);

                // Determine number of intervals for this tzId.
                int count = 0;
                foreach (NodaTime.TimeZones.ZoneInterval zi in intervals)
                    count++;

                bool singleEntry = (1 == count);

                // myStart and myEnd are desired output period
                // capture IsoLocalEnd of an interval to be used as the start date of program output (myStart).  Don't display older than 1900.
                DateTime myStart, myEnd, prevEnd = new DateTime(1900, 1, 1);

                foreach (NodaTime.TimeZones.ZoneInterval zi in intervals)
                {
                    if (singleEntry)
                    {
                        // No question here
                    }
                    else
                    {
                        // skip intervals for this tzId that represent a period beginning after time period I want.
                        if (instantValidityEnd < zi.Start)
                            break;

                        // Skip intervals ending prior to time period I want, but capture the IsoLocalEnd to be used for report output.
                        if (zi.End < instantValidityBegin)
                        {
                            prevEnd = (zi.End == Instant.MinValue) ? prevEnd = new DateTime(1900, 1, 1) : prevEnd = zi.IsoLocalEnd.ToDateTimeUnspecified(); ;
                            continue;
                        }

                        //   ***Question***  Can this myStart value be determined using the current interval record instead of storing the previous interval's IsoLocalEnd??
                        myStart = prevEnd;
                        if (zi.End == Instant.MaxValue)
                            prevEnd = myEnd = new DateTime(9999, 12, 31);
                        else
                        {
                            prevEnd = zi.IsoLocalEnd.ToDateTimeUnspecified();
                            myEnd = prevEnd.Subtract(TimeSpan.FromSeconds(1));  // force period back 1 second for logical representation.
                        }


                        sw.WriteLine("Name: " + zi.Name);
                        sw.WriteLine("Multi Entry: {0}", zi.ToString());
                        sw.WriteLine("myStart: {0:G}   myEnd: {1:G}    gmtOffset: {2:G}", myStart, myEnd, zi.WallOffset.ToTimeSpan());
                        sw.WriteLine();
                    }
                }

                sw.WriteLine("------------------------------------------------------------------");
                sw.WriteLine();
            }
        }
    }

Answer 1

On any given ZoneInterval record, the IsoLocalStart and IsoLocalEnd are relative to that particular interval, using the same WallOffset value for both.

If you want to get the actual local time in effect you can take the Start and End instants and convert them to the specific time zone.

zi.Start.InZone(zone)
zi.End.InZone(zone)

You'll find that the LocalDateTime of the starting value matches IsoLocalStart, but the ending value does not. By the time that instant is observed, it is now in covered by the offset of the next interval. Don't forget that these are half-open intervals. [inclusive-start, exclusive-end).

Let's go back to the example in your original request:

03/13/2016 2:00 am    -   11/06/2016  01:59:59 am  gmtOffset:  -4:00
11/06/2016 2:00 am    -   03/12/2017  01:59:59 am  gmtOffset:  -5:00

It doesn't actually work like that. In March, the local time moves from 1:59:59 am to 3:00:00 am, skipping over an hour. So the first row should start at 3:00 am rather than 2:00 am. In November, the local time moves from 01:59:59 am to 1:00:00 am repeating an hour. So the second row should start at 1:00 am rather than 2:00 am. Your chart seems to be attempting to make local time continuous - which it is not.

Additionally, you'll find that by convention many people think about the time of the transition as being 2:00, rather that 1:59:59. Think of it as approaching 2:00. This is why IsoLocalEnd is presented with the offset of the current interval, and you could use that field for such a purpose.

Also, you may want to check out how timeanddate.com shows it. They've decided to only show the ending values, which may make things a bit clearer.