The conversion could not be completed because the supplied DateTime did not have the Kind property set correctly

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

I solved by following

scheduledEmail.RegisteredDate = 
TimeZoneInfo.ConvertTimeToUtc(DateTime.SpecifyKind(scheduledEmail.RegisteredDate, DateTimeKind.Unspecified), tz);

But I don't know, why first does not work?

It depends on how the scheduledEmail.RegisteredDate was originated.

• If you got it from DateTime.Now, then it will have a Local kind.
• If you got it from DateTime.UtcNow then it will have an Utc kind.
• If you got it from new DateTime(2013,5,1) then it will have an Unspecified kind.

It also depends on where you got myTimeZone from. For example:

• TimeZoneInfo.Local
• TimeZoneInfo.Utc
• TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time")

The TimeZoneInfo.ConvertTimeToUtc function will only operate if it can match the zone to the kind you give it. If both are local, or both are UTC, then it will work. If you are giving it a specific zone, then the kind should be unspecified.