Develop Reference

Parse String into DateTimeOffset While Assuming TimeZone

Probably a super simple solution but I'm clearly missing something.
I have a string object with value "2020/07/29 13:30:00".
How can I parse that into a DateTimeOffset object and make the assumption that the time zone of that parsed time is "GMT Standard Time" for example, or any TimeZoneInfo I wish to specify preferably?
How can I then take that DateTimeOffset, and return its Utc time but to any specified time zone of my choice?
Many thanks

The easiest I could find is something like this.
I couldn't find any methods to parse a DateTimeOffset in a particular given timezone, but you can parse your string as a DateTime (with a Kind of Unspecified, which just acts as a container for the bits of information in the string, without trying to apply timezone knowledge to it).
Then you can ask a TimeZoneInfo for the UTC offset in a given timezone at the given local time, and apply this to the DateTime to create a DateTimeOffset.
Once you've got your DateTimeOffset, you can work with it using its ToOffset method, and TimeZoneInfo.ConvertTime.
string input = "2020/07/29 13:30:00";
var timezone = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
// DateTime.Parse creates a DateTime with Kind == Unspecified
var dateTime = DateTime.Parse(input);
Console.WriteLine(dateTime); // 7/29/2020 1:30:00 PM
// Since Kind == Unspecified, timezone.GetUtcOffset will give us the UTC offset in effect at
// the given local time in timezone
var dateTimeOffset = new DateTimeOffset(dateTime, timezone.GetUtcOffset(dateTime));
Console.WriteLine(dateTimeOffset); // 7/29/2020 1:30:00 PM +01:00
// Convert to UTC
Console.WriteLine(dateTimeOffset.UtcDateTime); // 7/29/2020 12:30:00 PM
Console.WriteLine(dateTimeOffset.ToOffset(TimeSpan.Zero)); // 7/29/2020 12:30:00 PM +00:00
// Convert to another timezone
var cst = TimeZoneInfo.FindSystemTimeZoneById("Central America Standard Time");
Console.WriteLine(TimeZoneInfo.ConvertTime(dateTimeOffset, cst)); // 7/29/2020 6:30:00 AM -06:00

Try the DateTimeOffset.ParseExact overload that accepts a DateTimeStyles parameter.
This code:
var dt=DateTimeOffset.ParseExact("2020/07/29 13:30:00","yyyy/MM/dd HH:mm:ss",
CultureInfo.InvariantCulture,DateTimeStyles.AssumeUniversal);
Returns 2020-07-29T13:30:00.0000000+00:00
There's no GMT Standard Time, that's a very unfortunate name used in Windows that somehow manages to mix up British and UTC time to the point that no-one knows what it means without looking at the docs. This was thoroughly discussed and explained in this question: Difference between UTC and GMT Standard Time in .NET. As one of the answers explains :
The names GMT Standard Time and GMT Daylight Time are unknown outside of Redmond. They are mythical animals that appear only in the bestiary called the Windows Registry.
If you wanted to assume British time and your machine uses a British timezone, you can use DateTimeStyles.AssumeLocal

This function should convert your date time string (with assumption this is GMT Standard Time) to any other timezone:
public static DateTime? ToUTCTimeZone(string sDate, string timeZone)
{
DateTime utcDate = DateTime.Parse(sDate);
DateTimeOffset localServerTime = DateTimeOffset.Now;
utcDate = DateTime.SpecifyKind(utcDate, DateTimeKind.Utc);
TimeZoneInfo cstZone = TimeZoneInfo.FindSystemTimeZoneById(timeZone);
if (cstZone == null)
return null;
return TimeZoneInfo.ConvertTimeFromUtc(utcDate, cstZone);
}//ToUTCTimeZone

Get time until local time is N hours from now, taking summer-time changes into account

Timezones and summer-time changes in particular confuse me. In the UK we have GMT/BST:
In the UK the clocks go forward 1 hour at 1am on the last Sunday in
March, and back 1 hour at 2am on the last Sunday in October. The
period when the clocks are 1 hour ahead is called British Summer Time
(BST).
Given a local time say 00:00 I want to be able to calculate how long until it is 03:00 in local time. Normally this is trivially 3 hours but on March 26th (last Sunday in March) from 00:00 - 03:00 is actually two hours. And similarly when the clocks go back in October from 00:00 - 03:00 is four hours.
Do the .Net DateTime class and its methods do this trivially for me or do I need to be careful?
In my case specifically I'm working from strings so I'm after a method doing:
TimeSpan DifferenceBetweenLocalTimes(string startDateTime,string endDateTime)
I can see things like TimeZoneInfo.IsDaylightSavingTime but how to use this to do as I wish is not obvious. My application works treating each calendar day's local midnight as a strict boundary i.e. not every day is 24 hours long, once a year I get a 23 hour day and a 25 hour day.

You can use TimeZoneInfo class to get the offset from your local date time to UTC (including daylight tricks). For example
var timeZone =TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
var date1 = DateTime.Parse("2017-03-26 00:00:00");
var date2 = DateTime.Parse("2017-03-26 03:00:00");
var dto1 = new DateTimeOffset(date1, timeZone.GetUtcOffset(date1));
var dto2 = new DateTimeOffset(date2, timeZone.GetUtcOffset(date2));
var diff1 = (dto2 - dto1).TotalHours;
Console.WriteLine(diff1); // this is 2 hours
The GetUtcOffset method returns difference between time in that time zone and UTC

While tchrikch's answer is perfectly reasonable (and should be accepted, IMHO), it's worth adding a Noda Time based solution.
// Parse input as LocalDateTime values (since they represent a local date and time)
var pattern = LocalDateTimePattern.CreateWithInvariantCulture("yyyy-MM-dd HH:mm:ss");
LocalDateTime ldt1 = pattern.Parse("2017-03-26 00:00:00").Value;
LocalDateTime ldt2 = pattern.Parse("2017-03-26 03:00:00").Value;
// Apply a specific time zone, now making them ZonedDateTime values
// Using "lenient" conversions allows for default handling of ambiguous/invalid values
DateTimeZone tz = DateTimeZoneProviders.Tzdb["Europe/London"];
ZonedDateTime zdt1 = ldt1.InZoneLeniently(tz);
ZonedDateTime zdt2 = ldt2.InZoneLeniently(tz);
// Now simply determine the elapsed duration between these
Duration result = zdt2 - zdt1;
Note that the subtraction between ZonedDateTime values was added in NodaTime 2.0. If you're on an older version, you'll need to do this instead:
Duration result = zdt2.ToInstant() - zdt1.ToInstant();

Convert current local time to any other timezone

I want to test some time-zone related code by comparing local time to UTC. However this test relies on local time being different to UTC and I'm in the UK so 6 months of the year, local time is UTC according to DateTime comparison tests (due to summer time).
I could hard-code my test to convert UTCNow to a certain timezone like EST but on the off-chance my code was used in an American system, now I have the same issue.
So is there a way I can easily convert DateTime.UtcNow to a timezone that's definitely different to my local timezone, without hard-coding it or making assumptions what timezone I'm in?

Ok, as I mentoined in comments, if you want to get timezones which differs from yours, you can do it in that way:
var zone = TimeZoneInfo.GetSystemTimeZones()
.Where(x=>x.BaseUtcOffset != TimeZoneInfo.Local.BaseUtcOffset)
.First();
To convert UTC DateTime to another timezone, you have to use TimeZoneInfo.ConvertTimeFromUtc, sample:
var datetime = // datetime in UTC, for example, DateTime.UtcNow
var zone = // target zone
return TimeZoneInfo.ConvertTimeFromUtc(datetime, zone);
You can check samples in my pet project here

Time zones in .net are a very confusing matter. DateTime only supports local and UTC timezones really, it has no concept of different zones as it only gets the current offset from the machine and applies it to a tick count value which is in UTC.
So at a first there's no way to change to another timezone, but you can simulate something. If you want per example simulate a timezone GMT+2, you must first retrieve the current zone offset and add the difference of this offset and the desired offset to the local date, something like this:
TimeSpan targetOffset = new TimeSpan(2, 0, 0) - TimeZoneInfo.Local.BaseUtcOffset; //target to GMT+2
DateTime targetNow = DateTime.Now + targetOffset;
In this way the Date will have the values like if it were on that timezone, but for calculations using the datetime object and not just the year/month/day/hour/minute/second all of them will be wrong as the object will be marked as local and thus will have the wrong values.
To solve this you have two options, reverse your logic (convert non-local time to local time) or just work with UTC dates.
The first approach is very easy:
DateTime nonLocal = new DateTime(2016, 10, 21, 13, 33, 0); //We suppose we want 2016-10-21 13:33:00 at GMT+2
DateTime local = nonLocal + (TimeZoneInfo.Local.BaseUtcOffset - new TimeSpan(2, 0, 0));
This will yield to correct DateTime operations, but, here we go again with problems, if you use the year/month/day/hour/minute they will be in your local zone, not in the supposed zone and code using these properties will fail.
Finally, the best approach is the second one, you just forget about timezones and whenever you get a DateTime you convert it to UTC, all will work flawlesly and when you need to represent the data just convert it to local and all is done, no need to worry about DateTime arithmetic or properties differing just because they're on another timezone.

public static class DateTimeExtensions
{
public static DateTime As(this DateTime source, string timeZoneName)
{
DateTime utcTime = DateTime.SpecifyKind(source, DateTimeKind.Unspecified);
TimeZoneInfo newTimeZone = TimeZoneInfo.FindSystemTimeZoneById(timeZoneName);
return TimeZoneInfo.ConvertTimeFromUtc(utcTime, newTimeZone);
}
}
Usage:
DateTime date1 = DateTime.UtcNow;
DateTime date2 = date1.As("Eastern Standard Time");

I want to specify a TimeZone as GMT but create a DateTime that is local (EST);

I want to specify a time in GMT timezone, and then convert it to the local TimeZone which is EST.
This appears to do what I want, but seems a long way to get there!
Is there a simpler way to achieve this:
public static TimeZoneInfo edtZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
public static TimeZoneInfo gmtZone = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
public static CultureInfo ci = CultureInfo.InvariantCulture;
DateTime edtStartDT = TimeZoneInfo.ConvertTime(DateTime.SpecifyKind(DateTime.Now.Date.Add(new TimeSpan(18, 00, 00)), DateTimeKind.Unspecified), gmtZone, edtZone);

This is probably what you are looking for:
// A timespan isn't really a time-of-day, but we'll let that slide for now.
TimeSpan time = new TimeSpan(18, 00, 00);
// Use the current utc date, at that time.
DateTime utcDateTime = DateTime.UtcNow.Date.Add(time);
// Convert to the US Eastern time zone.
TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
DateTime easternDateTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, tz);
Note that we're pairing the current UTC date with the time you provided. Since US Eastern time is either 5 or 4 hours behind UTC, with 18:00 you will always get the same date. But if you were using a different time, such as 00:00, you would find that the resulting Eastern time was on the prior day. This is normal.
A couple of notes about your previous code:
The Windows Time Zone ID "Eastern Standard Time" refers to both EST and EDT. It really should have been called "Eastern Time". Don't let the name confuse the issue.
GMT and UTC are mostly the same for all modern usage. Unless you are referring to the time zone used in London, you should prefer the term UTC.
The Windows Time Zone ID "GMT Standard Time" is not actually for GMT/UTC. It is specifically the time zone used in London, which alternates between GMT (UTC+00:00) and BST (UTC+01:00). If you want a TimeZoneInfo representing UTC, the ID is just "UTC". (However, you don't really need that in this case.)
Your original code assumed used DateTime.Now.Date, which would assume the date in the computer's local time zone - which might not be either UTC or Eastern.
If you find yourself using DateTime.SpecifyKind, in most situations, you're probably doing something wrong. (The exception would be when loading or deserializing.)
Regarding my note about a TimeSpan not being a true time-of-day, this is how .NET would have you handle that:
DateTime time = DateTime.Parse("18:00:00", CultureInfo.InvariantCulture);
DateTime utcDateTime = DateTime.UtcNow.Date.Add(time.TimeOfDay);
Or even uglier in one line:
DateTime utcDateTime = DateTime.Parse("18:00:00", CultureInfo.InvariantCulture,
DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal);
Personally, I would rather use Noda Time, which has a separate LocalTime type explicitly for a time-of-day that's not bound to a specific date. I'm also working to get System.TimeOfDate and System.Date types added to the CoreCLR.

Is there a way to make DateTime.IsDaylightSavingTime work on a machine with GMT/UTC time?

I'm trying to know if there is a way to make a DateTime object get all the right properties on a OS that is set to GMT time. Not just changing time.
Right now what im doing is:
DateTime f = new DateTime(2012, 8, 1); //A date on summer
DateTime f2 = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(f, TimeZoneInfo.Local.Id, "Central Europe Standard Time");
var isSummer = f2.IsDaylightSavingTime(); //It always returns false if the OS is on GMT
And it changes the hour & date to make the GMT become Central Europe Standard Time just fine, but the IsDaylightSavingTime function wont work. It always returns false because I'm using GMT on the system, but i think it shouldn't because I created a DateTime for central Europe.
Is there a way to make the DateTime object f2 really local and make it understand that if its August and is European Central it should return true instead of false?
I know if I use local time on the machine it would work OK, but I cant change the GMT time on the server. I would love to, but I cant. :-)

DateTime.IsDayLightSavingTime returns true if Kind is Local or Unspecified, and the value of this instance of DateTime is within the Daylight Saving Time range for the current time zone
I.e. DateTime object checks with system time zone, since UTC does not support daylight saving, it returns true.
If you want to verify, if the time specified fall in daylight saving for a time zone, then use IsDayLightSavingTime method in TimeZoneInfo
DateTime f = new DateTime(2012, 8, 1); //A date on summer
TimeZoneInfo tzf2=TimeZoneInfo.FindSystemTimeZoneById("Central Europe Standard Time");
DateTime f2 = TimeZoneInfo.ConvertTime(f, tzf2);
var isSummer = tzf2.IsDaylightSavingTime(f2);