I recommend using Noda Time to solve this. (Sure, you could use TimeZoneInfo but processing adjustment rules are a pain.)
Declare these imports:
using System;
using System.Linq;
using NodaTime;
Define a function to do the main work:
static (LocalDateTime StartDateTime, LocalDateTime UntilDateTime, Offset UtcOffset)[]
GetOffsetRanges(DateTimeZone tz, LocalDate startDate, LocalDate untilDate)
{
// Get ZonedDateTime values representing the start of day of each date in the given time zone.
ZonedDateTime startZonedDateTime = startDate.AtStartOfDayInZone(tz);
ZonedDateTime untilZonedDateTime = untilDate.AtStartOfDayInZone(tz);
// We'll also need the corresponding Instant of each value.
Instant startInstant = startZonedDateTime.ToInstant();
Instant untilInstant = untilZonedDateTime.ToInstant();
// Get the ZoneInterval values, which tell us the range that an offset is in effect.
return tz.GetZoneIntervals(startInstant, untilInstant).Select(interval =>
{
// Snap the values to the range we're working with. (This is optional, you could just use the IsoLocalStart/End values directly.)
LocalDateTime startDateTime = interval.Start >= startInstant ? interval.IsoLocalStart : startZonedDateTime.LocalDateTime;
LocalDateTime untilDateTime = interval.End <= untilInstant ? interval.IsoLocalEnd : untilZonedDateTime.LocalDateTime;
// Include the local date and time range, and the offset that's in effect over that range.
return (startDateTime, untilDateTime, interval.WallOffset);
}).ToArray();
}
Use the function like so:
// Define your input values
DateTimeZone tz = DateTimeZoneProviders.Tzdb["America/New_York"];
var startDate = new LocalDate(2020, 1, 1);
var untilDate = new LocalDate(2021, 1, 1);
// Call the function to get the ranges
var ranges = GetOffsetRanges(tz, startDate, untilDate);
// Example output
Console.WriteLine($"Offsets in {tz.Id} between {startDate:R} and {untilDate:R} are as follows:");
foreach (var range in ranges)
{
Console.WriteLine($"{range.StartDateTime:s} - {range.UntilDateTime:s} : {range.UtcOffset:m}");
}
Output:
Offsets in America/New_York between 2020-01-01 and 2021-01-01 are as follows:
2020-01-01T00:00:00 - 2020-03-08T02:00:00 : -05:00
2020-03-08T03:00:00 - 2020-11-01T02:00:00 : -04:00
2020-11-01T01:00:00 - 2021-01-01T00:00:00 : -05:00
A few notes:
I don't recommend simplifying the input or output in the way you stated in your question. It is not precise enough to say Mar-Oct.
As Panagiotis pointed out in the question comments, you will need to identify a time zone to use. A single offset input is not sufficient. My example showed a named IANA time zone, but if desired you could instead pass the system's local time zone by using DateTimeZoneProviders.Tzdb.GetSystemDefault()
If this is going to run on a server, then please don't use the system's local time zone. In general, the server's time zone should be considered irrelevant.
In a web application, you can get the user's IANA time zone identifier in JavaScript with Intl.DateTimeFormat().resolvedOptions().timeZone. You can pass that to your server and use it as input with the above code. (This works on most modern web browsers, but not all older ones.)
