How to tell if it's British Summer Time

Question

I have the following code, which should return an offset of 60 (to show that in the UK at present, we are in British Summer Time - ie. 60 minutes ahead of GMT):

Answer 1

Your machine is not configured correctly if you get 0 and live in the UK. Probable causes are:

• The machine's time zone is not set correctly. Click the clock on the taskbar to correct.
• The machine is not configured to observe daylight savings. Click the clock.
• The database that TimeZoneInfo consults for daylight savings rules in out of date or corrupted. It is stored in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones registry key.

Be careful about making radical changes, having it configured wrong might well have been done intentionally to work around some kind of flaw in a business-critical app that runs on the server. Talk to whomever administers the server first.

Answer 2

Another option is to use Noda Time.

The following code works with Noda Time 1.4 and higher:

var zone = NodaTime.TimeZones.TzdbDateTimeZoneSource.Default.ForId("Europe/London");
var zonedClock = NodaTime.SystemClock.Instance.InZone(zone);
var zonedDateTime = zonedClock.GetCurrentZonedDateTime();
bool isDST = zonedDateTime.IsDaylightSavingTime();
Console.WriteLine(isDST);

• zone is a DateTimeZone object representing the UK's timezone, "Europe/London"
• zonedClock is a ZonedClock object which represents, in this case, the system clock and the UK's timezone
• zonedDateTime is a ZonedDateTime object representing the current date and time in the timezone ("the current instant provided by the underlying clock, adjusted to the time zone of this object")
• isDST is a boolean indicating whether the current instant is in DST or not. At the time of writing (April 2018) this evaluates to true

---

With earlier versions of Noda where ZonedClock is not available, we can do this instead:

var zone = NodaTime.TimeZones.TzdbDateTimeZoneSource.Default.ForId("Europe/London");
var now = Instant.FromDateTimeOffset(DateTimeOffset.Now);
var zonedDateTime = new ZonedDateTime(now, zone);
bool isDST = zonedDateTime.IsDaylightSavingTime();

Answer 3

To master conversion from one timezone to anther you need to see what's supported (how?) and what's not.

foreach (var tz in TimeZoneInfo.GetSystemTimeZones())
{
    Console.WriteLine("TimeZone Offset: {0} Name : {1}, Supports DLS: {2}", tz.BaseUtcOffset,tz.StandardName,tz.SupportsDaylightSavingTime);
}

This should give you the list all timezones including info about DayLightSaving. Notice that:

TimeZone Offset: 00:00:00 Name : Greenwich Standard Time, Supports DLS: False

TimeZone Offset: 00:00:00 Name : GMT Standard Time, Supports DLS: True

So you need to use "GMT Standard Time" because it supports daylight saving already. No work needs to be done.

Here is sample code:

private static string GetBSTTimeStamp(string timestamp)
{
    DateTime dt = DateTime.Parse(timestamp);
    //TimeZoneInfo bst = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
    //Console.WriteLine("Time zone supports dls? {0}", bst.SupportsDaylightSavingTime);
    //Console.WriteLine("Time zone offset? {0}", bst.BaseUtcOffset);
    DateTime dateTimeInUtc = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(dt, "Eastern Standard Time", "GMT Standard Time");
    return dateTimeInUtc.ToString();
}

Answer 4

Use TimeZoneInfo.IsDaylightSavingTime Method (DateTimeOffset) to find if it is currently Daylight saving for your Timezone.

var info = TimeZoneInfo.FindSystemTimeZoneById("Greenwich Standard Time");
DateTimeOffset localServerTime = DateTimeOffset.Now;
bool isDaylightSaving = info.IsDaylightSavingTime(localServerTime);

There are further examples here