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Convert time data to local time in C#

I've read a few posts about similar subjects but nothing seems to answer this question. My database has the following information about a time
Day of the week (a number between 0-6)
Time (a number of milliseconds since midnight in the users local time)
UTC offset ( number of hours different to UTC )
DST Observed (boolean stating if DST is observed in that time zone)
This data represents opening hours. So there is a time for each day. I want to display that time in the users local time making the assumption that each day is in the future
int dayOffset = availability.Day - (int)now.DayOfWeek
if (dayOffset < 0)
dayOffset += 7;
I'm really struggling to get my head around time zones and handling when one time zone might be observing DST while another maybe DOES observe DST but hasn't yet.
My main issue at the moment is I think I need to create a DateTimeOffset object for the non-local time but I'm not sure how to do that as I don't know if DST is in effect or not.
I hope I'm making myself clear. It really is a mind-bending experience working with dates and time!

As indicated by other answers, the usual solution to handling DateTime across time zones would be to store UTC times.
However, considering that you are not referencing an absolute time at a specific date, but instead are referring to a time at an infinite number of days in a specific time zone; storing the time as an UTC time doesn't make sense anymore, since the UTC time (even if we discard the date) would be different depending on the date, due to DST.
The best way to store the time is fairly close to what you have done already.
Your problem is that the time zone information you are storing at the moment is ambiguous, as it does not refer to a specific time zone, but instead refers to properties of the time zone.
To solve this problem, simply store the time zone identifier instead of the UTC offset and DST boolean.
It is now possible for us to construct the DateTime object and convert it to any time zone by using the TimeZoneInfo class:
int dayOffset = availability.Day - (int)DateTime.Today.DayOfWeek;
if (dayOffset < 0)
{
dayOffset += 7;
}
var openingHourStart = DateTime
.SpecifyKind(DateTime.Today, DateTimeKind.Unspecified)
.AddDays(dayOffset)
.AddMilliseconds(availability.Time);
var sourceTimeZone = TimeZoneInfo.FindSystemTimeZoneById(availability.TimeZoneId);
var userTimeZone = TimeZoneInfo.Local;
var convertedOpeningHourStart = TimeZoneInfo.ConvertTime(openingHourStart,
sourceTimeZone,
userTimeZone);

Give a try to Quartz.NET.
It implements evaluation of CronExpressions, and even triggers to fire events at the given time. It can evaluate the next time an event will occur. This may help you out calculating the opening times.
Also, take a look at the cronmaker website: there you can understand the full potential of CronExpressions.
The CronExpressionDescriptor is a DotNET library for transforming CronExpressions into human readable strings.
Another library which I haven't tried yet is [HangFire].(https://www.hangfire.io/)
In this forum post you can find some discussion on how HangFire implements evaluation of RecurringJobs in local timezone with DST, which I believe is a solution for what you are looking for.

A comment to another answer made the problem a little bit more clear.
So, first and foremost, do store only UTC in your database. Really.
Now, since you are not interested in the actual dates, since you are storing working schedules that repeat weekly, the date only becomes relevant once you want to present your times - and when you put them in your database.
So let's first see how you get your times into your database correctly. I'm assuming a user will enter times in their own locale.
Make sure you first create a (localised) DateTime consisting of the current date and the given time (from the user), and transform that to a UTC DateTime (you can keep the current date, it doesn't matter):
var utcDateTime = DateTime.Now.Date
.AddHours(userHours)
.AddMinutes(userMinutes)
.ToUniversalTime();
Now when you are presenting these times to the user, simply go the other way:
var userDateTime = DateTime.Now.Date
.AddHours(utcDateTime.Hour)
.AddMinutes(utcDateTime.Minute)
.ToLocalTime();
And then you can use the userDateTime.Hour and .Minute for display purposes.

You should be leveraging DateTime.ToLocalTime() and TimeZoneInfo.ConvertTimeToUtc() in C# - see https://msdn.microsoft.com/en-us/library/system.datetime.tolocaltime(v=vs.110).aspx.
If you want to store only times that you're open from Monday to Sunday, fine. Have a simple data table to describe only the time for each day (0 = Sunday through 7 = Saturday -- this is .Net's DayOfWeek enumeration). Your lookup table might look like:
0 null
1 08:00:00
2 08:00:00
3 08:00:00
4 08:30:00
5 08:30:00
6 10:30:00
(Use whatever data type works for you--SQL Server 2008+ has a TIME data type, for example. Null can be used for Closed on that day--i.e., no open time.)
When it comes time to display YOUR time to any other user, use must create your UTC time on-the-fly at the moment you are displaying information to the local user.
Conyc provided one approach. My approach uses simple date/time strings. To use my approach, just store time values per day in your database. Then you can look up the open time for any given day. To express that time for another user in any locale, use this code to convert your time to UTC (you can substitute the "08:00:00 AM" string value with a string variable that you populated after looking up the open time in your database):
var StoreOpenTimeInUtc = TimeZoneInfo.ConvertTimeToUtc(Convert.ToDateTime("08:00:00 AM"));
To look up the open time in your database for a particular day in the future, you will need to concatenate the date to your time value, like this:
var StoreOpenTimeInUtc = TimeZoneInfo.ConvertTimeToUtc(Convert.ToDateTime("04/28/2018 08:00:00 AM"));
Once you have an accurate StoreOpenTimeInUtc variable, you can use that as the UTC value on someone else's machine who is anywhere else on planet earth. To convert that UTC value to their local time, use the .NET ToLocalTime() method:
var OpenTimeForLocalUser = StoreOpenTimeInUtc.ToLocalTime();
Note that this approach requires you to store only the open times as shown above. You don't have to worry about dates, local offsets from UTC, or anything else. Just leverage ConvertTimeToUtc() and ToLocalTime() as shown.

How do I accurately represent a Date Range in NodaTime?

Goals
I have a list of LocalDate items that represent sets of start dates and end dates.
I would like to be able to store date ranges, so that I can perform operations on them as a set of overlapping or distinct ranges, etc.
Questions
Does NodaTime provide some sort of DateRange construct that I've missed in the docs?
Am I thinking about this wrong? Is there a more natural / preferred way to accomplish this that NodaTime already allows for?
Am I setting myself up for trouble by attempting to think about a date range using a LocalDate for a start and an end date?
I'm completely new to NodaTime and assuming that this is a conceptual misunderstanding on my part.
Update: I noticed a similar question on the subject from 2009, but that seems to refer to another utilies class; I'm assuming that since then NodaTime may have evolved to accomodate this situation.

Noda Time provides an Interval type for a range of Instant values, but it doesn't provide range types for the other types. Part of the reason for this is the nuance of how ranges are used for different types.
If I give you a range of instants, it is always treated as a half open interval. The start value is included, but the end value is excluded. Humans do this naturally any time we provide a time value, such as when I say an event runs from 1:00 to 2:00, clearly I mean that the event is over at 2:00, so 2:00 is excluded.
But with whole calendar date ranges, typically the end dates are inclusive. To represent the entire month of January (as a range of LocalDate values), I would probably say Jan 1st through Jan 31st, and I am including the last day in its entirety.
We could probably add some additional range types to enforce these things, but we would need to think about how much value there is in having them in the API when you could probably just create them as needed. I'm not saying I'm for or against it either way, but that's probably something to be debated on the Noda Time user group.
To answer your specific questions:
No, there is no predefined range class for local dates.
The only other thing to consider is that calendar math is usually done via the Period class. For example, to determine how many days there are between two calendar dates:
LocalDate ld1 = new LocalDate(2012, 1, 1);
LocalDate ld2 = new LocalDate(2013, 12, 25);
Period period = Period.Between(ld1, ld2, PeriodUnits.Days);
long days = period.Days;
No, there's nothing wrong with creating a range class of local dates, there just might not be a whole lot of advantage. You may do just as well by having two properties, StartDate and EndDate, on your own classes. Just be careful about the inclusiveness of the end dates, vs the exclusiveness you'd see with an interval or time range.
And lastly, you said:
... so that I can perform operations on them as a set of overlapping or distinct ranges, etc.
You're probably looking for operations like intersection, union, calculating gaps, sorting, etc. These and more are defined by the Time Period Library, but Noda Time doesn't currently have anything like that. If one was to create it, it should probably be in a companion library ("NodaTime.Ranges", perhaps?). Likely it wouldn't be desired to pull it into the core, but you never know...
If you do end up using that Time Period Library, please make sure you recognize that it works with DateTime only, and is completely oblivious to DateTimeKind. So in order to be productive with it, you should probably make sure you are only working with UTC values, or "unspecified" calendar dates, and try not to ask it things like "how many hours are in a day" because it will get it wrong for days with daylight saving time transitions.

Representation of a DateTime as the local to remote user

Hello!
I was confused in the problem of time zones. I am writing a web application that will contain some news with dates of publication, and I want the client to see the date of publication of the news in the form of corresponding local time. However, I do not know in which time zone the client is located.
I have three questions.
I have to ask just in case: does DateTimeOffset.UtcNow always returns the correct UTC date and time, regardless of whether the server is dependent on daylight savings time? For example, if the first time I get the value of this property for two minutes before daylight savings time (or before the transition from daylight saving time back) and the second time in 2 minutes after the transfer, whether the value of properties in all cases differ by only 4 minutes? Or here require any further logic? (Question #1)
Please see the following example and tell me what you think.
I posted the news on the site. I assume that DateTimeOffset.UtcNow takes into account the time zone of the server and the daylight savings time, and so I immediately get the correct UTC server time when pressing the button "Submit". I write this value to a MS SQL database in the field of type datetime2(0).
Then the user opens a page with news and no matter how long after publication. This may occur even after many years. I did not ask him to enter his time zone. Instead, I get the offset of his current local time from UTC using the javascript function following way:
function GetUserTimezoneOffset()
{
var offset = new Date().getTimezoneOffset();
return offset;
}
Next I make the calculation of the date and time of publication, which will show the user:
public static DateTime Get_Publication_Date_In_User_Local_DateTime(
DateTime Publication_Utc_Date_Time_From_DataBase,
int User_Time_Zone_Offset_Returned_by_Javascript)
{
int userTimezoneOffset = User_Time_Zone_Offset_Returned_by_Javascript; // For
// example Javascript returns a value equal to -300, which means the
// current user's time differs from UTC to 300 minutes. Ie offset
// is UTC +6. In this case, it may be the time zone UTC +5 which
// currently operates summer time or UTC +6 which currently operates the
// standard time.
// Right? (Question #2)
DateTimeOffset utcPublicationDateTime =
new DateTimeOffset(Publication_Utc_Date_Time_From_DataBase,
TimeSpan.Zero); // get an instance of type DateTimeOffset for the
// date and time of publication for further calculations
DateTimeOffset publication_DateTime_In_User_Local_DateTime =
utcPublicationDateTime.ToOffset(new TimeSpan(0, - userTimezoneOffset, 0));
return publication_DateTime_In_User_Local_DateTime.DateTime;// return to user
}
Is the value obtained correct? Is this the right approach to solving this problem? (Question #3)
UPDATED Oct 19 at 6:58 (I tried post it as a comment but it's too long by 668 characters)
Matt Johnson, Thank You for such a detailed answer despite that of the fact that you are doing this not the first time. Thank you for taking the time to explain this particular case, and not just provide links to other posts.
I have read the information that you have provided. Maybe I'm still not fully aware of all the details, but if I understand it right, for the right convertion of DateTime (which was written many years ago in the database) from UTC to the same user's moment, I need to know UTC offset which he had at that moment. And it is difficult taking into account that transfer rules for DST change constantly. And even now, though the platform ".NET" contains some TZDB that is used for the TimeZoneInfo type, I can't take advantage of it without the exact position of the user.
But what if I am only interested in the date and time of starting this year, and only in Russia, where DST was canceled in 2011? As I understand it, this means that if properly configured clock on user's computer, located in Russia, this approach will always give the correct result. And since 2011, the offset to UTC of user's clock should always be the same. Accordingly, the shift indicators in different browsers will not be different for the Russian user's.

Answer to Question 1
... does DateTimeOffset.UtcNow always returns the correct UTC date and time, regardless of whether the server is dependent on daylight savings time?
Yes. As long is your clock is set correctly, UtcNow always refers to the UTC time. The time zone settings of the server will not affect it. The value in your example will always be 4 minutes, regardless of DST.
Answer to Question 2
var offset = new Date().getTimezoneOffset();
Since new Date() returns the current date and time, this will return you the current offset. You then proceed to apply this current offset to some past value, which may or may not be the correct offset for that specific time. Please read the timezone tag wiki, especially the section titled "Time Zone != Offset".
Answer to Question 3
Is the value obtained correct? Is this the right approach to solving this problem?
No. This is not the correct approach. You have a few options:
First Option
Just pass the UTC value to JavaScript without modification.
Send it in ISO8601 format, such as 2013-10-18T12:34:56.000Z. You can get this in .Net easily using yourDateTime.ToString("o").
Be sure the DateTime you are starting with has .Kind == DateTimeKind.Utc, otherwise it won't get the Z on the end, which is essential.
If you are targeting older browsers that can't parse ISO8601, then you will need a library such as moment.js to parse it for you.
Alternatively, you could pass the number of milliseconds since 1/1/1970 UTC as a number and load that into a JavaScript Date instead of parsing.
Now you can just display the Date object using JavaScript. Let the browser convert it from UTC to the users local time.
Warning, with this approach, some conversions might be incorrect due to the problem I describe here.
Second Option
Like the first option, pass the UTC timestamp to JavaScript
Use that to get the offset for that timestamp.
Pass the offset back to the server in a postback or ajax call.
Apply the offset on the server
Output the local time zone
I don't particularly like this option because of the round trip. You might as well calculate it in JavaScript like the first option.
Third Option
Get the user's time zone. The best way is to ask them for it, usually on a user setting page.
Use it to convert from the UTC time to their local time completely on the server.
You can use Windows time zones and the TimeZoneInfo class to do the conversions.
Or you can use IANA/Olson time zones and the Noda Time library.
If you do this, you can optionally use a map-based timezone picker.
You can take a guess at the user's time zone with jsTimeZoneDetect.
This approach is more accurate, but requires more user interaction.
Also, please in future posts, ask just one question at a time. And please search before you post - I've written most of this in one form or another many times before.

Add 1 week to current date

I've got something like this DateTime.Now.ToString("dd.MM.yy"); In my code, And I need to add 1 week to it, like 5.4.2012 to become 12.4.2012 I tried to convert it to int and then add it up, but there is a problem when it's up to 30.
Can you tell me some clever way how to do it?

You want to leave it as a DateTime until you are ready to convert it to a string.
DateTime.Now.AddDays(7).ToString("dd.MM.yy");

First, always keep the data in it's native type until you are ready to either display it or serialize it (for example, to JSON or to save in a file). You wouldn't convert two int variables to strings before adding or multiplying them, so don't do it with dates either.
Staying in the native type has a few advantages, such as storing the DateTime internally as 8 bytes, which is smaller than most of the string formats. But the biggest advantage is that the .NET Framework gives you a bunch of built in methods for performing date and time calculations, as well as parsing datetime values from a source string. The full list can be found here.
So your answer becomes:
Get the current timestamp from DateTime.Now. Use DateTime.Now.Date if you'd rather use midnight than the current time.
Use AddDays(7) to calculate one week later. Note that this method automatically takes into account rolling over to the next month or year, if applicable. Leap days are also factored in for you.
Convert the result to a string using your desired format
// Current local server time + 7 days
DateTime.Now.AddDays(7).ToString("dd.MM.yy");
// Midnight + 7 days
DateTime.Now.Date.AddDays(7).ToString("dd.MM.yy");
And there are plenty of other methods in the framework to help with:
Internationalization
UTC and timezones (though you might want to check out NodaTime for more advanced applications)
Operator overloading for some basic math calcs
The TimeSpan class for working with time intervals

Any reason you can't use the AddDays method as in
DateTime.Now.AddDays(7)

Get site region DateTime in ASP.NET

I need to get real DateTime by CultureInfo object. For example, my site is on US company server (that has own US time) but site is for Italy (example), where time is different, and on the site I should show Italy time, not US. In web.congig I have currentCulture="it-It"
Is it possible to get time for site and if so, what is the right way to do it?
Actually it would be great to have a function like:
public static DateTime GetSiteDate(CultureInfo ci)
{
....
return siteDate;
}
Thanks!

It is likely not possible to get the local time based on culture alone, mostly because time zones don't correspond one-to-one to countries. While Italy's time zone is Central European Standard Time (if I'm not mistaken), it will not always be the case that each country has a single time zone. For example, in the United States there are at least four time zones.

to find date DateTime.Now();
to find zone DateTime.Zone();
If you need much, there is no chance in ASP.NET you have to make use of Java Scripts to fulfill your needs.