Format Local DateTime with appropriate Local DST-aware Timezone (eg. PST or PDT) depending on time of year

Question

Given a Local DateTime value on a computer configured for PST (which will implicitly change to PDT on March 10th when DST kicks in), how can one obtain a string including the appropriate timezone - eg. PST/PDT, not offset! - in the output?

DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss ???")

Expected output strings, eg:

"2019-02-09 13:04:22 PST"  // right after lunch today
"2019-04-09 13:04:22 PDT"  // right after lunch in two months

The MSDN DateTime Custom Format Strings page shows examples of explicitly hard-coding "PST" into the output, which will be wrong half the year and/or when the local TZ is changed. Computers and people move, so hard-coding TZ values is simply 'not appropriate'.

Preferably this can be done with just a Format String, allowing DateTime values to be supplied until the rendering/to-string phase - although there does not appear to be a 'ZZZ' format. I've specified 'Local' DateTime Kind to, hopefully, reduce some additional quirks..

Answer 1

Since a DateTime instance does not keep timezone information, there is no way to do it with custom date and time format strings. "zzz" specifier is for UTC Offset value, DateTime.Kind with "K" specifier does not reflect time zone abbrevation neither. Both are useless for your case.

However, there is nuget package called TimeZoneNames which is written by time zone geek Matt Johnson that you can get abbreviations of a timezone name (supports both IANA and Windows time zone identifier)

var tz = TZNames.GetAbbreviationsForTimeZone("Pacific Standard Time", "en-US");
Console.WriteLine(tz.Standard); // PST
Console.WriteLine(tz.Daylight); // PDT

If you wanna get your windows time zone identifier programmatically, you can use TimeZoneInfo.Local.Id property, if you wanna get the current language code, you can use CultureInfo.CurrentCulture.Name property by the way.

var tz = TZNames.GetAbbreviationsForTimeZone(TimeZoneInfo.Local.Id, CultureInfo.CurrentCulture.Name);

But before that, you should check your local time is daylight saving time to choose which abbreviation to append your formatted string.

DateTime now = DateTime.Now;
bool isDaylight = TimeZoneInfo.Local.IsDaylightSavingTime(now);

If isDaylight is true, you should use the result of the TimeZoneValues.Daylight property, otherwise you should use TimeZoneValues.Standard property of the first code part.

At the end, you need append one of those abbreviation at the end of DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss) string.

For an important note from Matt on the package page;

Time zone abbreviations are sometimes inconsistent, and are not necessarily localized correctly for every time zone. In most cases, you should use abbreviations for end-user display output only. Do not attempt to use abbreviations when parsing input.

Second important note from Matt's comment;

What makes you think that a time zone abbreviation actually exists for every time zone and every language in the world? Even, then what makes you think that a time zone abbreviation is enough to identify the time zone? Hint - both questions are amorphous. Consider "CST" or "IST" - Each have three or four places in the world that they might belong to. Many many many other cases as well...

Answer 2

TimeZoneInfo should be able to help here: https://learn.microsoft.com/en-us/dotnet/api/system.timezoneinfo.standardname?view=netframework-4.7.2

It looks like TimeZoneInfo gives full names ("Pacific Standard Time"/"Pacific Daylight Time") rather than abbreviations ("PST"/"PDT"). It that's a problem, you'll still need to find a source for the short names. There are some ideas on how to do that here: Timezone Abbreviations

using System;
using Xunit;

namespace Q54610867
{
    public class TimeZoneTests
    {
        // I'm on Mac/Unix. If you're on Windows, change the ID to "Pacific Standard Time"
        // See: https://github.com/dotnet/corefx/issues/2538
        readonly TimeZoneInfo pacificStandardTime = TimeZoneInfo.FindSystemTimeZoneById("America/Los_Angeles");

        [Fact]
        public void Today()
        {
            var today = new DateTime(2019, 2, 9, 13, 4, 22, DateTimeKind.Local);

            Assert.Equal("2019-02-09 13:04:22 Pacific Standard Time", ToStringWithTz(today, pacificStandardTime));
        }

        [Fact]
        public void future()
        {
            var future = new DateTime(2019, 4, 9, 13, 4, 22, DateTimeKind.Local);

            Assert.Equal("2019-04-09 13:04:22 Pacific Daylight Time", ToStringWithTz(future, pacificStandardTime));
        }

        static string ToStringWithTz(DateTime dateTime, TimeZoneInfo tz)
            => $"{dateTime.ToString("yyyy-MM-dd HH:mm:ss")} {(tz.IsDaylightSavingTime(dateTime) ? tz.DaylightName : tz.StandardName)}";
    }
}