Daylight saving time with old dates

Question

In my system save all dates on UTC, but in the user view I show in GMT -3.

The problem is when I save a date in summer time, because in case it will no longer be made the difference of 3 hours, it saves by making the conversion correctly, however when I will recover that same date at a time other than in summer time it will return me with a different value, since when he saved was no longer the standard difference of 3 hours and at the time of the search is.

How can I solve this Daylight Saving Time problem by using C#.

Currently to do the conversions I use ToLocalTime() and ToUniversalTime().

Answer 1

There is confusion in the question and comments.

A time as we say informally is a point in time. It exists in a unique way. Time is something astrophysical, it has no time zone. That is why it is always right to treat points in time as UTC which is a universal time.

This site does this. And it’s the right one. In general websites have presentation problems because they don’t consider the spindle on the user’s machine. No big problem, the timing is right the presentation no, so this site chose not to try to guess the zone and uses UTC as presentation. I know a lot of people think it’s absurd, but this has its advantages, beyond the disadvantages.

In a more lay user application the ideal is to present the time within the zone. Presentation is something different from the time itself. Presentation is something visual, is eventual.

If the interest is the point in time does not matter much how the schedule entered or how it will present, only universal time matters. So if someone enters at 22:00 in GMT-0200 or 21:00 in GMT-0300 or 18:00 in GMT-0600, and then it makes no difference whether this is daylight saving or not, it’s time zone, everything will be 00:00UTC.

If the user’s machine is in the wrong zone is that machine’s problem. The most that can be done is for the system not to trust the operating system and to ask the user to inform what its spindle is. What is also unreliable, fool yourself even more. User makes confusion.

If something is set up wrong in the machine it is his problem. It is possible to create own alternatives to try to help.

Now, if you do not want a point in time then you need to save the time differently. If you want it visual time from the time the event occurred there record this time and not a point in time.

One way to do this is to record the time as local time same. You know that field is a visual representation of the time and not the time itself.

You can also continue recording UTC and have another field that tells you what use is used, so at the time of presenting you know how to calculate the presentation time.

Since this is not a point in time, it is not a quantification can even record a string with the schedule, it’s just a description.

If you have old hours in UTC and want to know what their time zone is without having it recorded, sorry, you have nothing to do, legacy error that will be loaded for the rest of your life. It’s like you use a integer and one day decide you wanted the decimal places of the recorded number.

Answer 2

If you are sure which zone the time was created in, you can use this method:

public static DateTime ConvertToBrazilianTime(DateTime utc)
{
    return TimeZoneInfo.ConvertTimeBySystemTimeZoneId
        (utc, "E. South America Standard Time"); // Horário de Brasilia/BRA
}

This method receives a UTC date and converts it to the specified zone, it is intelligent sufficient to identify whether daylight saving time was in effect at the specified place and date.

Example:

// Data UTC 15/12/2012 10:00:00
DateTime dataUtc = MinhaQueryDoBanco.DataUtc;

// Converte de UTC para horário de Brasilia
// O método sabe que nessa data (15/12/2012) o horário de verão estava vigente
// então o datetime é convertido para 15/12/2012 08:00:00
// usando -02:00 ao invés de -03:00
DateTime dataLocal = ConvertToBrazilianTime(dataUtc);

See working on . NET Fiddle

Example 2:

var date = new List<DateTime>();

// Criando Lista com range de datas.
for (int i = 0; i < 10000; i++)
{
    date.Add(DateTime.UtcNow.AddHours(-i));
}

foreach (var d in date)
{
    Console.WriteLine(ConvertToBrazilianTime(d));
} 

Result (note the difference on day 16 when daylight saving time comes into effect):

See working on . NET Fiddle