Having problems with converting my DateTime to UTC

I am storing all my dates in UTC format in my database. I ask the user for their timezone and I want to use their time zone plus what I am guessing is the server time to figure out the UTC for them.

Once I have that I want to do a search to see what the range is in the database using their newly converted UTC date.

But I always get this exception.

System.ArgumentException was unhandled by user code  
Message="The conversion could not be completed because the   
supplied DateTime did not have the Kind property set correctly.  
For example, when the Kind property is DateTimeKind.Local,   
the source time zone must be TimeZoneInfo.Local.  
Parameter name: sourceTimeZone"

I don't know why I am getting this.

I tried 2 ways

 TimeZoneInfo zone = TimeZoneInfo.FindSystemTimeZoneById(id);
 // I also tried DateTime.UtcNow
 DateTime now = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Local); 
 var utc = TimeZoneInfo.ConvertTimeToUtc(now , zone );

This failed so I tried

 DateTime now = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Local); 
 var utc = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(now, 
                                           ZoneId, TimeZoneInfo.Utc.Id);

This also failed with the same error. What am I doing wrong?

Edit Would this work?

 DateTime localServerTime = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Local);
 TimeZoneInfo info = TimeZoneInfo.FindSystemTimeZoneById(id);

 var usersTime = TimeZoneInfo.ConvertTime(localServerTime, info);

 var utc = TimeZoneInfo.ConvertTimeToUtc(usersTime, userInfo);

Edit 2 @ Jon Skeet

Yes, I was just thinking about that I might not even need to do all this. Time stuff confuses me right now so thats why the post may not be as clear as it should be. I never know what the heck DateTime.Now is getting (I tried to change my Timezone to another timezone and it kept getting my local time).

This is what I wanted to achieve: User comes to the site, adds some alert and it gets saved as utc (prior it was DateTime.Now, then someone suggested to store everything UTC).

So before a user would come to my site and depending where my hosting server was it could be like on the next day. So if the alert was said to be shown on August 30th (their time) but with the time difference of the server they could come on August 29th and the alert would be shown.

So I wanted to deal with that. So now I am not sure should I just store their local time then use this offset stuff? Or just store UTC time. With just storing UTC time it still might be wrong since the user still probably would be thinking in local time and I am not sure how UTC really works. It still could end up in a difference of time.

Edit3

 var info = TimeZoneInfo.FindSystemTimeZoneById(id)

 DateTimeOffset usersTime = TimeZoneInfo.ConvertTime(DataBaseUTCDate,
                                             TimeZoneInfo.Utc, info);

You need to set the Kind to Unspecified, like this:

DateTime now = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Unspecified);
var utc = TimeZoneInfo.ConvertTimeToUtc(now , zone);

DateTimeKind.Local means the in local time zone, and not any other time zone. That's why you were getting the error.

The DateTime structure supports only two timezones:

• The local timezone the machine is running in.
• and UTC.

Have a look at the DateTimeOffset structure.

var info = TimeZoneInfo.FindSystemTimeZoneById("Tokyo Standard Time");

DateTimeOffset localServerTime = DateTimeOffset.Now;

DateTimeOffset usersTime = TimeZoneInfo.ConvertTime(localServerTime, info);

DateTimeOffset utc = localServerTime.ToUniversalTime();

Console.WriteLine("Local Time:  {0}", localServerTime);
Console.WriteLine("User's Time: {0}", usersTime);
Console.WriteLine("UTC:         {0}", utc);

Output:

Local Time:  30.08.2009 20:48:17 +02:00
User's Time: 31.08.2009 03:48:17 +09:00
UTC:         30.08.2009 18:48:17 +00:00

Everyone else's answer seems overly complex. I had a specific requirement and this worked fine for me:

void Main()
{
	var startDate = DateTime.Today;
	var StartDateUtc = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(DateTime.SpecifyKind(startDate.Date, DateTimeKind.Unspecified), "Eastern Standard Time", "UTC");
	startDate.Dump();
	StartDateUtc.Dump();
}

Which outputs (from linqpad) what I expected:

12/20/2013 12:00:00 AM

12/20/2013 5:00:00 AM

Props to Slaks for the Unspecified kind tip. That's what I was missing. But all the talk about there being only two kinds of dates (local and UTC) just muddled the issue for me.

FYI -- the machine I ran this on was in Central Time Zone and DST was not in effect.

As dtb says, you should use DateTimeOffset if you want to store a date/time with a specific time zone.

However, it's not at all clear from your post that you really need to. You only give examples using DateTime.Now and you say you're guessing that you're using the server time. What time do you actually want? If you just want the current time in UTC, use DateTime.UtcNow or DateTimeOffset.UtcNow. You don't need to know the time zone to know the current UTC time, precisely because it's universal.

If you're getting a date/time from the user in some other way, please give more information - that way we'll be able to work out what you need to do. Otherwise we're just guessing.

UTC is just a time zone that everyone agreed on as the standard time zone. Specifically, it's a time zone that contains London, England. EDIT: Note that it's not the exact same time zone; for example, UTC has no DST. (Thanks, Jon Skeet)

The only special thing about UTC is that it's much easier to use in .Net than any other time zone (DateTime.UtcNow, DateTime.ToUniversalTime, and other members).

Therefore, as others have mentioned, the best thing for you to do is store all dates in UTC within your database, then convert to the user's local time (by writing TimeZoneInfo.ConvertTime(time, usersTimeZone) before displaying.

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If you want to be fancier, you can geolocate your users' IP addresses to automatically guess their time zones.