Develop Reference

DateTimeKind is Unspecified Even Though I Set it

When I check optionDate's DateTime property's DateTimeKind value, I see Unspecified, even though I set dt's DateTimeKind as UTC in below code. I expect optionDate has a DateTime which has a DateTimeKind property set to UTC. Where am I wrong here?
var dt = new DateTime(Convert.ToInt32(optionDateInfo.dateTime.year),
Convert.ToInt32(optionDateInfo.dateTime.month), Convert.ToInt32(optionDateInfo.dateTime.day),
Convert.ToInt32(optionDateInfo.dateTime.hour), Convert.ToInt32(optionDateInfo.dateTime.minutes),
0, DateTimeKind.Utc);
var optionDate = new DateTimeOffset(dt);

This is documented:
DateTimeOffset.DateTime
The value of the DateTime.Kind property of the returned DateTime object is DateTimeKind.Unspecified.
Note that a DateTimeOffset does not have a "kind". It has a date, time, and offset. When you pass your DateTime with kind Utc, to it, it sets its offset to 0, and its date & time to the DateTime given. At this point, your DateTimeKind is "lost".
An offset of 0 does not necessarily mean that its kind is DateTimeKind.Utc. It could be the local time in London, or somewhere in Africa too. So it can't give you a DateTime with kind Utc just because its offset is 0 either.
In addition, DateTime being able to represent 3 kinds of things is already a questionable design, and if the DateTime property can now return 3 different kinds of DateTime depending on whether offset matches the local time, is UTC, or something else, that's just even worse.
Instead, it is designed to have 3 properties that give you DateTimes with different kinds.
DateTime gives you the date & time part of the DateTimeOffset, with kind Unspecified
LocalDateTime converts the date & time part of the DateTimeOffset to the current timezone, and gives you a DateTime with kind Local.
UtcDateTime converts the date & time part of the DateTimeOffset to UTC, and gives you a DateTime with kind Utc.
If you want a DateTime with kind Utc, you should use that last one.

Use the SpecifyKind
var myUtcZeroOffset = DateTime.SpecifyKind(DateTime.UtcNow, DateTimeKind.Utc)
//If constructing a datetime offset to be not utc you can supply the offset instead
var myOffSetExplicitLocal = new DateTimeOffset(DateTime.Now, new TimeSpan(1, 0, 0));
var localDateTime = myOffSetExplicitLocal.DateTime;
var utcZeroOffSetDateTime = myOffSetExplicitLocal.UtcDateTime;
To make matters worse of cause it is a criticisable implementation from Microsoft, because Universally Coordinated Time is not a timezone but a notation, as per ISO 8601, so in fact toUTC as a concept is flawed because '2021-11-02T10:16:25.12345+01:00' is completely valid in the UTC format and UTC Zero offset, popularily called Zulu being the '2021-11-02T09:16:25.12345Z' equivalent which then gets datetimekind UTC is actually just in coordinated time the zero line around GMT latitude, but what makes it coordinated is the + part which in +00:00 can be abbreviated to Z, so lots of stuff is done to mitigate the inherent conflict and with build servers and cloud providers the .Local is especially dubious, so I would recommend always to persist in ISO 8601 strings instead, unless you actually need to use them in with date operations in Your DB, in said case to name fields appropriate like DateTimeCreatedUtcZero column e.g.
just my five cents of reason on the topic in general, hope it helps.

Converting to epoch time-stamp adding hours offset

I have date time in a string as "20160127003500".
What I need to do is convert this to Unix time-stamp adding hours to it.
I want to add hours offset as "1" or "2" or "24".
Can anyone please guide me in right direction.
Regards

First, parse the entire string (including the offset you mentioned in the question comments) to a DateTimeOffset:
using System.Globalization;
string s = "20160129205500 +0100";
string format = "yyyyMMddHHmmss zzz";
DateTimeOffset dto = DateTimeOffset.ParseExact(s, format, CultureInfo.InvariantCulture);
Then, there are a few different ways to get a Unix timestamp. Note that by the pure definition of a "Unix timestamp", the result would be in terms of seconds, though many languages these days use a higher precision (such as milliseconds used in JavaScript).
If you are targeting .NET 4.6 or higher, simply use the built-in methods:
// pick one for the desired precision:
long timestamp = dto.ToUnixTimeMilliseconds();
long timestamp = dto.ToUnixTimeSeconds();
If you are targeting an earlier version of .NET, then compute it yourself:
DateTime epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
// pick one for the desired precision:
long timestamp = (long) dto.UtcDateTime.Subtract(epoch).TotalMilliseconds;
long timestamp = (long) dto.UtcDateTime.Subtract(epoch).TotalSeconds;

How do I stop a date/time comparison from failing when a user is in a different time zone?

I am getting the "LastWriteTime" of my executable and comparing it to an internal DateTime that I have set. If the LastWriteTime is less than or equal to the internal DateTime then I will clear two tables from a database.
This code works great for me in the Pacific Time Zone. But if a user is in another time zone, (example 4 hours ahead of me), then it does not work because the "LastWriteTime" returns the time converted to their time zone. For example, I am looking for the value of "12/12/2012 8:38:12 AM" and if they are 4 hours ahead of me, this value gets automatically changed to "12/12/2012 12:38:12 PM" on their systems.
Can someone please show me what I should modify in my code to take into account for different time zones so the "LastWriteTime" and my 'build2023_EXE_Date' variable both return the same Date/Time so my comparision of the two date/time values don't fail regardless of what time zone my end user is in?
I am using .NET 3.5, not .Net 4.x
//http://stackoverflow.com/questions/1600962/displaying-the-build-date
string w_file = "MyEXE.exe";
string w_directory = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) +
Path.DirectorySeparatorChar + "MyEXE";
DateTime currentExeTime = File.GetLastWriteTime(System.IO.Path.Combine(w_directory, w_file));
DateTime build2023_EXE_Date = new DateTime(2012, 12, 12, 8, 38, 12); //"12/12/2012 8:38:12 AM"
//We need to truncate the millisecond time off of the EXE LastWriteTime
//or else when we compare to our internal DateTime build2323_EXE_Date value,
//it will not match
//http://stackoverflow.com/questions/1004698/how-to-truncate-milliseconds-off-of-a-net-datetime
currentExeTime = new DateTime(
currentExeTime.Ticks - (currentExeTime.Ticks % TimeSpan.TicksPerSecond),
currentExeTime.Kind
);
if (currentExeTime <= build2023_EXE_Date) //If previous build matches or is before the Build 2023 date then clear these two tables.
{
//This will fail the comparision if the user is in a different time zone than me.
//Clear tables
}

Unless you've got a specific need to keep dates in local time or have an associated time zone, I suggest you use universal time instead. This makes working with dates far easier because they all compare sanely, and it can actually be more performant (when you request DateTime.Now, .NET calls DateTime.UtcNow and then performs a relatively expensive adjustment to local time).
Another option is to use DateTimeOffset, which stores a date with an offset (not a time zone! -- for instance, DST will give you different offsets) and makes comparisons as easy as a universal DateTime. Unfortunately, though, GetLastWriteTime doesn't use DateTimeOffset so this might not work for you.

Use the DateTime ToUniversalTime() method

C#: Making sure DateTime.Now returns a GMT + 1 time

I am using DateTime.Now to show something according to today's date, and when working locally (Malta, Europe) the times appear correctly (obviously because of the Time Zone) but ofcourse when I upload it to my hosting server (USA), DateTime.Now does not represent the correct time zone.
Therefore, in my code, how can I convert DateTime.Now to correctly return the time from a GMT + 1 timezone ?

Use the TimeZoneInfo class found in System.Core;
You must set the DateTimeKind to DateTimeKind.Utc for this.
DateTime MyTime = new DateTime(1990, 12, 02, 19, 31, 30, DateTimeKind.Utc);
DateTime MyTimeInWesternEurope = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(MyTime, "W. Europe Standard Time");
Only if you're using .Net 3.5 though!

It depends on what you mean by "a GMT + 1 timezone". Do you mean permanently UTC+1, or do you mean UTC+1 or UTC+2 depending on DST?
If you're using .NET 3.5, use TimeZoneInfo to get an appropriate time zone, then use:
// Store this statically somewhere
TimeZoneInfo maltaTimeZone = TimeZoneInfo.FindSystemTimeZoneById("...");
DateTime utc = DateTime.UtcNow;
DateTime malta = TimeZoneInfo.ConvertTimeFromUtc(utc, maltaTimeZone );
You'll need to work out the system ID for the Malta time zone, but you can do that easily by running this code locally:
Console.WriteLine(TimeZoneInfo.Local.Id);
Judging by your comments, this bit will be irrelevant, but just for others...
If you're not using .NET 3.5, you'll need to work out the daylight savings yourself. To be honest, the easiest way to do that is going to be a simple lookup table. Work out the DST changes for the next few years, then write a simple method to return the offset at a particular UTC time with that list hardcoded. You might just want a sorted List<DateTime> with the known changes in, and alternate between 1 and 2 hours until your date is after the last change:
// Be very careful when building this list, and make sure they're UTC times!
private static readonly IEnumerable<DateTime> DstChanges = ...;
static DateTime ConvertToLocalTime(DateTime utc)
{
int hours = 1; // Or 2, depending on the first entry in your list
foreach (DateTime dstChange in DstChanges)
{
if (utc < dstChange)
{
return DateTime.SpecifyKind(utc.AddHours(hours), DateTimeKind.Local);
}
hours = 3 - hours; // Alternate between 1 and 2
}
throw new ArgumentOutOfRangeException("I don't have enough DST data!");
}

I don't think that you can set a property in your code that will make DateTime.Now return anything else than the current time of the computer in which the code executes. If you want to have a way of always getting another time, you will probably need to wrap in another function. You can can do the round-trip over UTC and add the desired offset:
private static DateTime GetMyTime()
{
return DateTime.UtcNow.AddHours(1);
}
(code sample updated after Luke's comment on the inner workings of DateTime.Now)

Timestamp as UTC Integer

I Have a legacy database with a field containing an integer representing a datetime in UTC
From the documentation:
"Timestamps within a CDR appear in Universal Coordinated Time (UTC). This value remains
independent of daylight saving time changes"
An example of a value is 1236772829.
My question is what is the best way to convert it to a .NET DateTime (in CLR code, not in the DB), both as the UTC value and as a local time value.
Have tried to google it but without any luck.

You'll need to know what the integer really means. This will typically consist of:
An epoch/offset (i.e. what 0 means) - for example "midnight Jan 1st 1970"
A scale, e.g. seconds, milliseconds, ticks.
If you can get two values and what they mean in terms of UTC, the rest should be easy. The simplest way would probably be to have a DateTime (or DateTimeOffset) as the epoch, then construct a TimeSpan from the integer, e.g. TimeSpan.FromMilliseconds etc. Add the two together and you're done. EDIT: Using AddSeconds or AddMilliseconds as per aakashm's answer is a simpler way of doing this bit :)
Alternatively, do the arithmetic yourself and call the DateTime constructor which takes a number of ticks. (Arguably the version taking a DateTimeKind as well would be better, so you can explicitly state that it's UTC.)

Googling that exact phrase gives me this Cicso page, which goes on to say "The field specifies a time_t value that is obtained from the operating system. "
time_t is a C library concept which strictly speaking doesn't have to be any particular implementation, but typically for UNIX-y systems will be the number of seconds since the start of the Unix epoch, 1970 January 1 00:00.
Assuming this to be right, this code will give you a DateTime from an int:
DateTime epochStart = new DateTime(1970, 1, 1);
int cdrTimestamp = 1236772829;
DateTime result = epochStart.AddSeconds(cdrTimestamp);
// Now result is 2009 March 11 12:00:29
You should sanity check the results you get to confirm that this is the correct interpretation of these time_ts.