在 DateTime 和 DateTimeOffset 之間轉換
更新：2007 年 11 月
雖然 DateTimeOffset 結構對時區的感知程度要比 DateTime 結構高，但在方法呼叫中較常使用的卻是 DateTime 參數。因此，知道如何將 DateTimeOffset 值與 DateTime 值相互轉換，就十分重要。本主題說明如何執行這些轉換，同時盡可能保留時區資訊。
.gif "注意事項") 注意事項： |
|---|
無論是使用 DateTime 或 DateTimeOffset 型別來表示時區中的時間，都會受到某些限制。透過 DateTime 的 Kind 屬性，只能反映 Coordinated Universal Time (UTC) 和系統的當地時區。DateTimeOffset 雖然可以反映時間的 UTC 位移，但無法反映位移所屬的實際時區為何。如需時間值和時區支援的詳細資訊，請參閱在 DateTime、DateTimeOffset 和 TimeZoneInfo 之間選擇。 |
從 DateTime 轉換為 DateTimeOffset
DateTimeOffset 結構提供兩種對等的方法來執行 DateTime 至 DateTimeOffset 的轉換，這兩種方法可處理大部分的轉換：
DateTimeOffset 建構函式，會根據 DateTime 值來建立新的 DateTimeOffset 物件。
隱含轉換運算子，可以讓您將 DateTime 值指派給 DateTimeOffset 物件。
針對 UTC 和本地 DateTime 值，所產生之 DateTimeOffset 值的 Offset 屬性會準確地反映出 UTC 或本地時區位移。例如，下列程式碼會將 UTC 時間轉換成其對等的 DateTimeOffset 值。
Dim utcTime1 As Date = Date.SpecifyKind(#06/19/2008 7:00AM#, _
DateTimeKind.Utc)
Dim utcTime2 As DateTimeOffset = utcTime1
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}", _
utcTime1, _
utcTime1.Kind.ToString(), _
utcTime2)
' This example displays the following output to the console:
' Converted 6/19/2008 7:00:00 AM Utc to a DateTimeOffset value of 6/19/2008 7:00:00 AM +00:00
DateTime utcTime1 = new DateTime(2008, 6, 19, 7, 0, 0);
utcTime1 = DateTime.SpecifyKind(utcTime1, DateTimeKind.Utc);
DateTimeOffset utcTime2 = utcTime1;
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}",
utcTime1,
utcTime1.Kind.ToString(),
utcTime2);
// This example displays the following output to the console:
// Converted 6/19/2008 7:00:00 AM Utc to a DateTimeOffset value of 6/19/2008 7:00:00 AM +00:00
在此例中，utcTime2 變數的位移為 00:00。同樣地，下列程式碼會將本地時間轉換成其對等的 DateTimeOffset 值。
Dim localTime1 As Date = Date.SpecifyKind(#06/19/2008 7:00AM#, DateTimeKind.Local)
Dim localTime2 As DateTimeOffset = localTime1
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}", _
localTime1, _
localTime1.Kind.ToString(), _
localTime2)
' This example displays the following output to the console:
' Converted 6/19/2008 7:00:00 AM Local to a DateTimeOffset value of 6/19/2008 7:00:00 AM -07:00
DateTime localTime1 = new DateTime(2008, 6, 19, 7, 0, 0);
localTime1 = DateTime.SpecifyKind(localTime1, DateTimeKind.Local);
DateTimeOffset localTime2 = localTime1;
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}",
localTime1,
localTime1.Kind.ToString(),
localTime2);
// This example displays the following output to the console:
// Converted 6/19/2008 7:00:00 AM Local to a DateTimeOffset value of 6/19/2008 7:00:00 AM -07:00
不過，對於 Kind 屬性為 DateTimeKind.Unspecified 的 DateTime 值，這兩種轉換方法都會產生其位移為本地時區位移的 DateTimeOffset 值。這會在下列範例中示範，此範例執行的時區為美國太平洋標準時間。
Dim time1 As Date = #06/19/2008 7:00AM# ' Kind is DateTimeKind.Unspecified
Dim time2 As DateTimeOffset = time1
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}", _
time1, _
time1.Kind.ToString(), _
time2)
' This example displays the following output to the console:
' Converted 6/19/2008 7:00:00 AM Unspecified to a DateTimeOffset value of 6/19/2008 7:00:00 AM -07:00
DateTime time1 = new DateTime(2008, 6, 19, 7, 0, 0); // Kind is DateTimeKind.Unspecified
DateTimeOffset time2 = time1;
Console.WriteLine("Converted {0} {1} to a DateTimeOffset value of {2}",
time1,
time1.Kind.ToString(),
time2);
// This example displays the following output to the console:
// Converted 6/19/2008 7:00:00 AM Unspecified to a DateTimeOffset value of 6/19/2008 7:00:00 AM -07:00
如果 DateTime 值反映的日期和時間不是本地時區或 UTC，您可以藉由呼叫多載 DateTimeOffset 建構函式，將它轉換成 DateTimeOffset 值並保留其時區資訊。例如，下列範例會將反映中央標準時間的 DateTimeOffset 物件具現化。
Dim time1 As Date = #06/19/2008 7:00AM# ' Kind is DateTimeKind.Unspecified
Try
Dim time2 As New DateTimeOffset(time1, _
TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(time1))
Console.WriteLine("Converted {0} {1} to a DateTime value of {2}", _
time1, _
time1.Kind.ToString(), _
time2)
' Handle exception if time zone is not defined in registry
Catch e As TimeZoneNotFoundException
Console.WriteLine("Unable to identify target time zone for conversion.")
End Try
' This example displays the following output to the console:
' Converted 6/19/2008 7:00:00 AM Unspecified to a DateTime value of 6/19/2008 7:00:00 AM -05:00
DateTime time1 = new DateTime(2008, 6, 19, 7, 0, 0); // Kind is DateTimeKind.Unspecified
try
{
DateTimeOffset time2 = new DateTimeOffset(time1,
TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(time1));
Console.WriteLine("Converted {0} {1} to a DateTime value of {2}",
time1,
time1.Kind.ToString(),
time2);
}
// Handle exception if time zone is not defined in registry
catch (TimeZoneNotFoundException)
{
Console.WriteLine("Unable to identify target time zone for conversion.");
}
// This example displays the following output to the console:
// Converted 6/19/2008 7:00:00 AM Unspecified to a DateTime value of 6/19/2008 7:00:00 AM -05:00
此建構函式多載的第二個參數 (即表示時間之 UTC 位移的 TimeSpan 物件) 應藉由呼叫時間相對應時區的 TimeZoneInfo.GetUtcOffset(DateTime) 方法來擷取。這個方法的單一參數就是 DateTime 值，此值表示要轉換的日期和時間。如果時區支援日光節約時間，則此參數可讓方法判斷該特定日期和時間的適當位移。
從 DateTimeOffset 轉換為 DateTime
DateTime 屬性最常用來執行 DateTimeOffset 至 DateTime 的轉換。不過，它會傳回 Kind 屬性為 Unspecified 的 DateTime 值，如下列範例所示。
Const baseTime As Date = #06/19/2008 7:00AM#
Dim sourceTime As DateTimeOffset
Dim targetTime As Date
' Convert UTC to DateTime value
sourceTime = New DateTimeOffset(baseTime, TimeSpan.Zero)
targetTime = sourceTime.DateTime
Console.WriteLine("{0} converts to {1} {2}", _
sourceTime, _
targetTime, _
targetTime.Kind.ToString())
' Convert local time to DateTime value
sourceTime = New DateTimeOffset(baseTime, _
TimeZoneInfo.Local.GetUtcOffset(baseTime))
targetTime = sourceTime.DateTime
Console.WriteLine("{0} converts to {1} {2}", _
sourceTime, _
targetTime, _
targetTime.Kind.ToString())
' Convert Central Standard Time to a DateTime value
Try
Dim offset As TimeSpan = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(baseTime)
sourceTime = New DateTimeOffset(baseTime, offset)
targetTime = sourceTime.DateTime
Console.WriteLine("{0} converts to {1} {2}", _
sourceTime, _
targetTime, _
targetTime.Kind.ToString())
Catch e As TimeZoneNotFoundException
Console.WriteLine("Unable to create DateTimeOffset based on U.S. Central Standard Time.")
End Try
' This example displays the following output to the console:
' 6/19/2008 7:00:00 AM +00:00 converts to 6/19/2008 7:00:00 AM Unspecified
' 6/19/2008 7:00:00 AM -07:00 converts to 6/19/2008 7:00:00 AM Unspecified
' 6/19/2008 7:00:00 AM -05:00 converts to 6/19/2008 7:00:00 AM Unspecified
DateTime baseTime = new DateTime(2008, 6, 19, 7, 0, 0);
DateTimeOffset sourceTime;
DateTime targetTime;
// Convert UTC to DateTime value
sourceTime = new DateTimeOffset(baseTime, TimeSpan.Zero);
targetTime = sourceTime.DateTime;
Console.WriteLine("{0} converts to {1} {2}",
sourceTime,
targetTime,
targetTime.Kind.ToString());
// Convert local time to DateTime value
sourceTime = new DateTimeOffset(baseTime,
TimeZoneInfo.Local.GetUtcOffset(baseTime));
targetTime = sourceTime.DateTime;
Console.WriteLine("{0} converts to {1} {2}",
sourceTime,
targetTime,
targetTime.Kind.ToString());
// Convert Central Standard Time to a DateTime value
try
{
TimeSpan offset = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(baseTime);
sourceTime = new DateTimeOffset(baseTime, offset);
targetTime = sourceTime.DateTime;
Console.WriteLine("{0} converts to {1} {2}",
sourceTime,
targetTime,
targetTime.Kind.ToString());
}
catch (TimeZoneNotFoundException)
{
Console.WriteLine("Unable to create DateTimeOffset based on U.S. Central Standard Time.");
}
// This example displays the following output to the console:
// 6/19/2008 7:00:00 AM +00:00 converts to 6/19/2008 7:00:00 AM Unspecified
// 6/19/2008 7:00:00 AM -07:00 converts to 6/19/2008 7:00:00 AM Unspecified
// 6/19/2008 7:00:00 AM -05:00 converts to 6/19/2008 7:00:00 AM Unspecified
這表示當使用 DateTime 屬性時，在轉換之後，關於 DateTimeOffset 值與 UTC 之間關係的任何資訊都將會遺失。這會影響對應至 UTC 時間或系統本地時間的 DateTimeOffset 值，因為 DateTime 結構的 Kind 屬性中只會反映這兩種時區。
為了在轉換 DateTimeOffset 為 DateTime 值時盡可能保留時區資訊，您可以使用 DateTimeOffset.UtcDateTime 和 DateTimeOffset.LocalDateTime 屬性。
轉換 UTC 時間
若要指示轉換的 DateTime 值是 UTC 時間，您可以擷取 DateTimeOffset.UtcDateTime 屬性的值。它在下列兩方面與 DateTime 屬性不同：
| 注意事項： |
|---|
如果您的應用程式需要轉換的 DateTime 值明確識別單一時間點，請考慮使用 DateTimeOffset.UtcDateTime 屬性來處理所有 DateTimeOffset 至 DateTime 的轉換。 |
下列程式碼使用 UtcDateTime 屬性來將位移等於 Zero() 的 DateTimeOffset 值轉換為 DateTime 值。
Dim utcTime1 As New DateTimeOffset(#06/19/2008 7:00AM#, TimeSpan.Zero)
Dim utcTime2 As Date = utcTime1.UtcDateTime
Console.WriteLine("{0} converted to {1} {2}", _
utcTime1, _
utcTime2, _
utcTime2.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM +00:00 converted to 6/19/2008 7:00:00 AM Utc
DateTimeOffset utcTime1 = new DateTimeOffset(2008, 6, 19, 7, 0, 0, TimeSpan.Zero);
DateTime utcTime2 = utcTime1.UtcDateTime;
Console.WriteLine("{0} converted to {1} {2}",
utcTime1,
utcTime2,
utcTime2.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM +00:00 converted to 6/19/2008 7:00:00 AM Utc
下列程式碼使用 UtcDateTime 屬性來對 DateTimeOffset 值同時執行時區轉換和型別轉換。
Dim originalTime As New DateTimeOffset(#6/19/2008 7:00AM#, _
New TimeSpan(5, 0, 0))
Dim utcTime As Date = originalTime.UtcDateTime
Console.WriteLine("{0} converted to {1} {2}", _
originalTime, _
utcTime, _
utcTime.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM +05:00 converted to 6/19/2008 2:00:00 AM Utc
DateTimeOffset originalTime = new DateTimeOffset(2008, 6, 19, 7, 0, 0, new TimeSpan(5, 0, 0));
DateTime utcTime = originalTime.UtcDateTime;
Console.WriteLine("{0} converted to {1} {2}",
originalTime,
utcTime,
utcTime.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM +05:00 converted to 6/19/2008 2:00:00 AM Utc
轉換本地時間
若要指示 DateTimeOffset 值表示的是本地時間，您可以將 DateTimeOffset.DateTime 屬性傳回的 DateTime 值傳遞至 static (在 Visual Basic 中為 Shared) SpecifyKind() 方法。這個方法會傳回第一個參數傳遞給它的日期和時間，但其 Kind 屬性會被設為第二個參數所指定的值。下列程式碼使用 SpecifyKind() 方法來轉換值相當於本地時區位移的 DateTimeOffset。
Dim sourceDate As Date = #06/19/2008 7:00AM#
Dim utcTime1 As New DateTimeOffset(sourceDate, _
TimeZoneInfo.Local.GetUtcOffset(sourceDate))
Dim utcTime2 As Date = utcTime1.DateTime
If utcTime1.Offset.Equals(TimeZoneInfo.Local.GetUtcOffset(utcTime1.DateTime)) Then
utcTime2 = DateTime.SpecifyKind(utcTime2, DateTimeKind.Local)
End If
Console.WriteLine("{0} converted to {1} {2}", _
utcTime1, _
utcTime2, _
utcTime2.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
DateTime sourceDate = new DateTime(2008, 6, 19, 7, 0, 0);
DateTimeOffset utcTime1 = new DateTimeOffset(sourceDate,
TimeZoneInfo.Local.GetUtcOffset(sourceDate));
DateTime utcTime2 = utcTime1.DateTime;
if (utcTime1.Offset.Equals(TimeZoneInfo.Local.GetUtcOffset(utcTime1.DateTime)))
utcTime2 = DateTime.SpecifyKind(utcTime2, DateTimeKind.Local);
Console.WriteLine("{0} converted to {1} {2}",
utcTime1,
utcTime2,
utcTime2.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
您也可以使用 DateTimeOffset.LocalDateTime 屬性來將 DateTimeOffset 值轉換為本地 DateTime 值。傳回之 DateTime 值的 Kind 屬性為 Local。下列程式碼會使用 DateTimeOffset.LocalDateTime 屬性來轉換值相當於本地時區位移的 DateTimeOffset 值。
Dim sourceDate As Date = #06/19/2008 7:00AM#
Dim localTime1 As New DateTimeOffset(sourceDate, _
TimeZoneInfo.Local.GetUtcOffset(sourceDate))
Dim localTime2 As Date = localTime1.LocalDateTime
Console.WriteLine("{0} converted to {1} {2}", _
localTime1, _
localTime2, _
localTime2.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
DateTime sourceDate = new DateTime(2008, 6, 19, 7, 0, 0);
DateTimeOffset localTime1 = new DateTimeOffset(sourceDate,
TimeZoneInfo.Local.GetUtcOffset(sourceDate));
DateTime localTime2 = localTime1.LocalDateTime;
Console.WriteLine("{0} converted to {1} {2}",
localTime1,
localTime2,
localTime2.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
當您使用 DateTimeOffset.LocalDateTime 屬性擷取 DateTime 值時，屬性的 get 存取子會先將 DateTimeOffset 值轉換成 UTC，然後再藉由呼叫 ToLocalTime 方法將它轉換成本地時間。這表示您可以從 DateTimeOffset.LocalDateTime 屬性擷取值來執行時區轉換，並且同時執行型別轉換。這也表示會套用本地時區的調整規則來執行轉換。下列程式碼說明如何使用 DateTimeOffset.LocalDateTime 屬性來同時執行型別和時區轉換。
Dim originalDate As DateTimeOffset
Dim localDate As Date
' Convert time originating in a different time zone
originalDate = New DateTimeOffset(#06/19/2008 7:00AM#, _
New TimeSpan(-5, 0, 0))
localDate = originalDate.LocalDateTime
Console.WriteLine("{0} converted to {1} {2}", _
originalDate, _
localDate, _
localDate.Kind.ToString())
' Convert time originating in a different time zone
' so local time zone's adjustment rules are applied
originalDate = New DateTimeOffset(#11/04/2007 4:00AM#, _
New TimeSpan(-5, 0, 0))
localDate = originalDate.LocalDateTime
Console.WriteLine("{0} converted to {1} {2}", _
originalDate, _
localDate, _
localDate.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM -05:00 converted to 6/19/2008 5:00:00 AM Local
' 11/4/2007 4:00:00 AM -05:00 converted to 11/4/2007 1:00:00 AM Local
DateTimeOffset originalDate;
DateTime localDate;
// Convert time originating in a different time zone
originalDate = new DateTimeOffset(2008, 6, 18, 7, 0, 0,
new TimeSpan(-5, 0, 0));
localDate = originalDate.LocalDateTime;
Console.WriteLine("{0} converted to {1} {2}",
originalDate,
localDate,
localDate.Kind.ToString());
// Convert time originating in a different time zone
// so local time zone's adjustment rules are applied
originalDate = new DateTimeOffset(2007, 11, 4, 4, 0, 0,
new TimeSpan(-5, 0, 0));
localDate = originalDate.LocalDateTime;
Console.WriteLine("{0} converted to {1} {2}",
originalDate,
localDate,
localDate.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM -05:00 converted to 6/19/2008 5:00:00 AM Local
// 11/4/2007 4:00:00 AM -05:00 converted to 11/4/2007 1:00:00 AM Local
通用轉換方法
在下列範例中，會定義名為 ConvertFromDateTimeOffset 的方法，用它來將 DateTimeOffset 值轉換成 DateTime 值。根據其位移而定，它會判斷 DateTimeOffset 值是 UTC 時間、本地時間或其他時間，然後依此定義所傳回日期和時間值的 Kind 屬性。
Function ConvertFromDateTimeOffset(dateTime As DateTimeOffset) As Date
If dateTime.Offset.Equals(TimeSpan.Zero) Then
Return dateTime.UtcDateTime
ElseIf dateTime.Offset.Equals(TimeZoneInfo.Local.GetUtcOffset(dateTime.DateTime))
Return Date.SpecifyKind(dateTime.DateTime, DateTimeKind.Local)
Else
Return dateTime.DateTime
End If
End Function
static DateTime ConvertFromDateTimeOffset(DateTimeOffset dateTime)
{
if (dateTime.Offset.Equals(TimeSpan.Zero))
return dateTime.UtcDateTime;
else if (dateTime.Offset.Equals(TimeZoneInfo.Local.GetUtcOffset(dateTime.DateTime)))
return DateTime.SpecifyKind(dateTime.DateTime, DateTimeKind.Local);
else
return dateTime.DateTime;
}
下列範例會呼叫 ConvertFromDateTimeOffset 方法來轉換表示 UTC 時間、本地時間以及美國中央標準時區時間的 DateTimeOffset 值。
Dim timeComponent As Date = #06/19/2008 7:00AM#
Dim returnedDate As Date
' Convert UTC time
Dim utcTime As New DateTimeOffset(timeComponent, TimeSpan.Zero)
returnedDate = ConvertFromDateTimeOffset(utcTime)
Console.WriteLine("{0} converted to {1} {2}", _
utcTime, _
returnedDate, _
returnedDate.Kind.ToString())
' Convert local time
Dim localTime As New DateTimeOffset(timeComponent, _
TimeZoneInfo.Local.GetUtcOffset(timeComponent))
returnedDate = ConvertFromDateTimeOffset(localTime)
Console.WriteLine("{0} converted to {1} {2}", _
localTime, _
returnedDate, _
returnedDate.Kind.ToString())
' Convert Central Standard Time
Dim cstTime As New DateTimeOffset(timeComponent, _
TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(timeComponent))
returnedDate = ConvertFromDateTimeOffset(cstTime)
Console.WriteLine("{0} converted to {1} {2}", _
cstTime, _
returnedDate, _
returnedDate.Kind.ToString())
' The example displays the following output to the console:
' 6/19/2008 7:00:00 AM +00:00 converted to 6/19/2008 7:00:00 AM Utc
' 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
' 6/19/2008 7:00:00 AM -05:00 converted to 6/19/2008 7:00:00 AM Unspecified
DateTime timeComponent = new DateTime(2008, 6, 19, 7, 0, 0);
DateTime returnedDate;
// Convert UTC time
DateTimeOffset utcTime = new DateTimeOffset(timeComponent, TimeSpan.Zero);
returnedDate = ConvertFromDateTimeOffset(utcTime);
Console.WriteLine("{0} converted to {1} {2}",
utcTime,
returnedDate,
returnedDate.Kind.ToString());
// Convert local time
DateTimeOffset localTime = new DateTimeOffset(timeComponent,
TimeZoneInfo.Local.GetUtcOffset(timeComponent));
returnedDate = ConvertFromDateTimeOffset(localTime);
Console.WriteLine("{0} converted to {1} {2}",
localTime,
returnedDate,
returnedDate.Kind.ToString());
// Convert Central Standard Time
DateTimeOffset cstTime = new DateTimeOffset(timeComponent,
TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time").GetUtcOffset(timeComponent));
returnedDate = ConvertFromDateTimeOffset(cstTime);
Console.WriteLine("{0} converted to {1} {2}",
cstTime,
returnedDate,
returnedDate.Kind.ToString());
// The example displays the following output to the console:
// 6/19/2008 7:00:00 AM +00:00 converted to 6/19/2008 7:00:00 AM Utc
// 6/19/2008 7:00:00 AM -07:00 converted to 6/19/2008 7:00:00 AM Local
// 6/19/2008 7:00:00 AM -05:00 converted to 6/19/2008 7:00:00 AM Unspecified
請注意，此程式碼做了兩項假設，但根據應用程式以及其日期和時間值的來源，這些假設不一定會成立：
它假設位移為 Zero 的日期和時間值是代表 UTC。事實上，UTC 並不是特定時區的時間，而是相對於全球時區中已標準化的時間。時區也可以有 Zero 的位移。
它假設位移等於本地時區位移的日期和時間是代表本地時區。由於日期和時間值與其原始時區並不相關，因此假設不一定成立。日期和時間可能來自具有相同位移的另一個時區。