Handling UTC time within C#?

I am not an expert on the internals of C# so this question might actually be downright silly. If so, please correct me. I have some data (in UTC format) taken out from a SQL server.

2011-03-26 11:03:58.000
2011-03-26 11:04:25.000
...

I am parsing this file inside C# and am using the following:

DateTime date = DateTime.Parse(value);

to fetch the value into the DateTimeobject. Now, I am subtracting some arbitrary time 6 hours from this time as follows:

date = date.Subtract(new TimeSpan(6, 0, 0));

And finally, I am writing this back into another file as follows:

output.WriteLine(date.ToString("yyyy-MM-dd HH:mm:ss.fff"));

Because I have not done any implicit conversions, the output is also UTC. My question is, is this kind of addition/subtraction of time allowed on the parsed date or do I need to do some UTC to C# specific conve开发者_运维百科rsion before being able to manipulate time? I am having difficulties wrapping my head around this. Would someone please clarify this?

EDIT: Attempt to write a concise question The original date is in UTC. I want to add/subtract some time and write it back in UTC. I want to know if I can manipulate the parsed date directly or I need to do some conversions i.e. tell C# explicitly that the date is in UTC format and then manipulate it and then tell it again to write back UTC date.

It doesn't really matter. A date is a date, no matter what time zone it's in. Unless you're converting it to a different time zone, .Net doesn't know or care what time zone it is.

If you want to, you can call DateTime.SpecifyKind(value, DateTimeKind.Utc).

You do not need to parse datetimes from SQL Server. The ADO.Net will return a SqlDateTime object for a datetime type column in a result. ORM libraries (LinqToSQL, Entity Framework etc) are also perfectly capable of mapping datetime type columns to DateTime properties. If you find yourself parsing a string, you're doing it wrong (not to mention all the implications of SET DATEFORMAT...)

As a note the operation you described can be performed straight on the server, eg. add one hour to an UTC datetime field and save it back as UTC datetime:

UPDATE table SET column = DATEADD(hour, 1, column) WHERE ...

Is the original date the UTC time, or your local time? If the original is local time and you are subtracting 8 hours to make it UTC, then use DateTime.ToUniversalTime(). To go the other way, use DateTime.ToLocalTime(). Or am I missing something?

Use DateTimeOffset

With the input string being universal, you can use an overload of DateTime.Parse that accepts a DateTimeStyle to indicate this.

string utcDateString = "2011-03-26 11:03:58.000";

DateTime localDate = DateTime.Parse(utcDateString, 
                          CultureInfo.CurrentCulture, 
                          DateTimeStyles.AssumeUniversal);

DateTime utcDate = localDate.ToUniversalTime();

In this snippet, localDate would be the local time of the universal time input. So for EDT (US), it would be 7:03:58 AM. utcDate would have 11:03:58, matching the input string.