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How to: Round-trip Date and time values

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In many applications, a date and time value is intended to unambiguously identify a single point in time. This article shows how to save and restore a DateTime value, a DateTimeOffset value, and a date and time value with time zone information so that the restored value identifies the same time as the saved value.

Round-trip a DateTime value

  1. Convert the DateTime value to its string representation by calling the DateTime.ToString(String) method with the "o" format specifier.

  2. Save the string representation of the DateTime value to a file, or pass it across a process, application domain, or machine boundary.

  3. Retrieve the string that represents the DateTime value.

  4. Call the DateTime.Parse(String, IFormatProvider, DateTimeStyles) method, and pass DateTimeStyles.RoundtripKind as the value of the styles parameter.

The following example illustrates how to round-trip a DateTime value.

const string fileName = @".\DateFile.txt";

StreamWriter outFile = new StreamWriter(fileName);

// Save DateTime value.
DateTime dateToSave = DateTime.SpecifyKind(new DateTime(2008, 6, 12, 18, 45, 15),
                                           DateTimeKind.Local);
string? dateString = dateToSave.ToString("o");
Console.WriteLine("Converted {0} ({1}) to {2}.",
                  dateToSave.ToString(),
                  dateToSave.Kind.ToString(),
                  dateString);
outFile.WriteLine(dateString);
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName);
outFile.Close();

// Restore DateTime value.
DateTime restoredDate;

using StreamReader inFile = new StreamReader(fileName);
dateString = inFile.ReadLine();

if (dateString is not null)
{
    restoredDate = DateTime.Parse(dateString, null, DateTimeStyles.RoundtripKind);
    Console.WriteLine("Read {0} ({2}) from {1}.", restoredDate.ToString(),
                                                  fileName,
                                                  restoredDate.Kind.ToString());
}

// The example displays the following output:
//    Converted 6/12/2008 6:45:15 PM (Local) to 2008-06-12T18:45:15.0000000-05:00.
//    Wrote 2008-06-12T18:45:15.0000000-05:00 to .\DateFile.txt.
//    Read 6/12/2008 6:45:15 PM (Local) from .\DateFile.txt.

Const fileName As String = ".\DateFile.txt"

Dim outFile As New StreamWriter(fileName)

' Save DateTime value.
Dim dateToSave As Date = DateTime.SpecifyKind(#06/12/2008 6:45:15 PM#, _
                                              DateTimeKind.Local)
Dim dateString As String = dateToSave.ToString("o")
Console.WriteLine("Converted {0} ({1}) to {2}.", dateToSave.ToString(), _
                  dateToSave.Kind.ToString(), dateString)
outFile.WriteLine(dateString)
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName)
outFile.Close()

' Restore DateTime value.
Dim restoredDate As Date

Dim inFile As New StreamReader(fileName)
dateString = inFile.ReadLine()
inFile.Close()
restoredDate = DateTime.Parse(dateString, Nothing, DateTimeStyles.RoundTripKind)
Console.WriteLine("Read {0} ({2}) from {1}.", restoredDate.ToString(), _
                  fileName, restoredDAte.Kind.ToString())
' The example displays the following output:
'    Converted 6/12/2008 6:45:15 PM (Local) to 2008-06-12T18:45:15.0000000-05:00.
'    Wrote 2008-06-12T18:45:15.0000000-05:00 to .\DateFile.txt.
'    Read 6/12/2008 6:45:15 PM (Local) from .\DateFile.txt.

When round-tripping a DateTime value, this technique successfully preserves the time for all local and universal times. For example, if a local DateTime value is saved on a system in the U.S. Pacific Standard Time zone and is restored on a system in the U.S. Central Standard Time zone, the restored date and time will be two hours later than the original time, which reflects the time difference between the two time zones. However, this technique is not necessarily accurate for unspecified times. All DateTime values whose Kind property is Unspecified are treated as if they are local times. If it's not a local time, the DateTime doesn't successfully identify the correct point in time. The workaround for this limitation is to tightly couple a date and time value with its time zone for the save and restore operation.

Round-trip a DateTimeOffset value

  1. Convert the DateTimeOffset value to its string representation by calling the DateTimeOffset.ToString(String) method with the "o" format specifier.

  2. Save the string representation of the DateTimeOffset value to a file, or pass it across a process, application domain, or machine boundary.

  3. Retrieve the string that represents the DateTimeOffset value.

  4. Call the DateTimeOffset.Parse(String, IFormatProvider, DateTimeStyles) method, and pass DateTimeStyles.RoundtripKind as the value of the styles parameter.

The following example illustrates how to round-trip a DateTimeOffset value.

const string fileName = @".\DateOff.txt";

StreamWriter outFile = new StreamWriter(fileName);

// Save DateTime value.
DateTimeOffset dateToSave = new DateTimeOffset(2008, 6, 12, 18, 45, 15,
                                               new TimeSpan(7, 0, 0));
string? dateString = dateToSave.ToString("o");
Console.WriteLine("Converted {0} to {1}.", dateToSave.ToString(),
                  dateString);
outFile.WriteLine(dateString);
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName);
outFile.Close();

// Restore DateTime value.
DateTimeOffset restoredDateOff;

using StreamReader inFile = new StreamReader(fileName);
dateString = inFile.ReadLine();

if (dateString is not null)
{
    restoredDateOff = DateTimeOffset.Parse(dateString, null,
                                           DateTimeStyles.RoundtripKind);
    Console.WriteLine("Read {0} from {1}.", restoredDateOff.ToString(), fileName);
}

// The example displays the following output:
//    Converted 6/12/2008 6:45:15 PM +07:00 to 2008-06-12T18:45:15.0000000+07:00.
//    Wrote 2008-06-12T18:45:15.0000000+07:00 to .\DateOff.txt.
//    Read 6/12/2008 6:45:15 PM +07:00 from .\DateOff.txt.

Const fileName As String = ".\DateOff.txt"

Dim outFile As New StreamWriter(fileName)

' Save DateTime value.
Dim dateToSave As New DateTimeOffset(2008, 6, 12, 18, 45, 15, _
                                     New TimeSpan(7, 0, 0))
Dim dateString As String = dateToSave.ToString("o")
Console.WriteLine("Converted {0} to {1}.", dateToSave.ToString(), dateString)
outFile.WriteLine(dateString)
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName)
outFile.Close()

' Restore DateTime value.
Dim restoredDateOff As DateTimeOffset

Dim inFile As New StreamReader(fileName)
dateString = inFile.ReadLine()
inFile.Close()
restoredDateOff = DateTimeOffset.Parse(dateString, Nothing, DateTimeStyles.RoundTripKind)
Console.WriteLine("Read {0} from {1}.", restoredDateOff.ToString(), fileName)
' The example displays the following output:
'    Converted 6/12/2008 6:45:15 PM +07:00 to 2008-06-12T18:45:15.0000000+07:00.
'    Wrote 2008-06-12T18:45:15.0000000+07:00 to .\DateOff.txt.
'    Read 6/12/2008 6:45:15 PM +07:00 from .\DateOff.txt.

This technique always unambiguously identifies a DateTimeOffset value as a single point in time. The value can then be converted to Coordinated Universal Time (UTC) by calling the DateTimeOffset.ToUniversalTime method, or it can be converted to the time in a particular time zone by calling the DateTimeOffset.ToOffset or TimeZoneInfo.ConvertTime(DateTimeOffset, TimeZoneInfo) method. The major limitation of this technique is that date and time arithmetic, when performed on a DateTimeOffset value that represents the time in a particular time zone, may not produce accurate results for that time zone. This is because when a DateTimeOffset value is instantiated, it is disassociated from its time zone. Therefore, that time zone's adjustment rules can no longer be applied when you perform date and time calculations. You can work around this problem by defining a custom type that includes both a date and time value and its accompanying time zone.

Compile the code

These examples require that the following namespaces be imported with C# using directives or Visual Basic Imports statements:

See also