Code:
/ FX-1434 / FX-1434 / 1.0 / untmp / whidbey / REDBITS / ndp / clr / src / BCL / System / TimeZone.cs / 1 / TimeZone.cs
// ==++== // // Copyright (c) Microsoft Corporation. All rights reserved. // // ==--== /*============================================================ ** ** Class: TimeZone ** ** ** Purpose: ** This class is used to represent a TimeZone. It ** has methods for converting a DateTime to UTC from local time ** and to local time from UTC and methods for getting the ** standard name and daylight name of the time zone. ** ** The only TimeZone that we support in version 1 is the ** CurrentTimeZone as determined by the system timezone. ** ** ============================================================*/ namespace System { using System; using System.Text; using System.Threading; using System.Collections; using System.Globalization; [Serializable] [System.Runtime.InteropServices.ComVisible(true)] public abstract class TimeZone { private static TimeZone currentTimeZone = null; // Private object for locking instead of locking on a public type for SQL reliability work. private static Object s_InternalSyncObject; private static Object InternalSyncObject { get { if (s_InternalSyncObject == null) { Object o = new Object(); Interlocked.CompareExchange(ref s_InternalSyncObject, o, null); } return s_InternalSyncObject; } } protected TimeZone() { } public static TimeZone CurrentTimeZone { get { //Grabbing the cached value is required at the top of this function so that //we don't incur a race condition with the ResetTimeZone method below. TimeZone tz = currentTimeZone; if (tz == null) { lock(InternalSyncObject) { if (currentTimeZone == null) { currentTimeZone = new CurrentSystemTimeZone(); } tz = currentTimeZone; } } return (tz); } } //This method is called by CultureInfo.ClearCachedData in response to control panel //change events. It must be synchronized because otherwise there is a race condition //with the CurrentTimeZone property above. internal static void ResetTimeZone() { if (currentTimeZone!=null) { lock(InternalSyncObject) { currentTimeZone = null; } } } public abstract String StandardName { get; } public abstract String DaylightName { get; } public abstract TimeSpan GetUtcOffset(DateTime time); // // Converts the specified datatime to the Universal time base on the current timezone // public virtual DateTime ToUniversalTime(DateTime time) { if (time.Kind == DateTimeKind.Utc) { return time; } long tickCount = time.Ticks - GetUtcOffset(time).Ticks; if (tickCount>DateTime.MaxTicks) { return new DateTime(DateTime.MaxTicks, DateTimeKind.Utc); } if (tickCount03:00) 10:00 -8:00 [This time doesn't actually exist, but it can be created from DateTime] // 03:00 10:00 -7:00 // 04:00 11:00 -7:00 // 05:00 12:00 -7:00 // // So from 02:00 - 02:59:59, we should return the standard offset, instead of the daylight saving offset. // // In the transition from daylight saving time to standard time, // if we convert local time to Universal time, we can have the // following (take PST as an example): // Local Universal UTC Offset // ----- --------- ---------- // 01:00AM 08:00 -7:00 // 02:00 (=> 01:00) 09:00 -8:00 // 02:00 10:00 -8:00 // 03:00 11:00 -8:00 // 04:00 12:00 -8:00 // // So in this case, the 02:00 does exist after the first 2:00 rolls back to 01:00. We don't need to special case this. // But note that there are two 01:00 in the local time. // // And imagine if the daylight saving offset is negative (although this does not exist in real life) // In the transition from standard time to daylight saving time, // if we convert local time to Universal time, we can have the // following (take PST as an example, but the daylight saving offset is -01:00): // Local Universal UTC Offset // ----- --------- ---------- // 01:00AM 09:00 -8:00 // 02:00 (=> 01:00) 10:00 -9:00 // 02:00 11:00 -9:00 // 03:00 12:00 -9:00 // 04:00 13:00 -9:00 // 05:00 14:00 -9:00 // // So in this case, the 02:00 does exist after the first 2:00 rolls back to 01:00. We don't need to special case this. // // In the transition from daylight saving time to standard time, // if we convert local time to Universal time, we can have the // following (take PST as an example, daylight saving offset is -01:00): // // Local Universal UTC Offset // ----- --------- ---------- // 01:00AM 10:00 -9:00 // 02:00 (=> 03:00) 11:00 -9:00 // 03:00 11:00 -8:00 // 04:00 12:00 -8:00 // 05:00 13:00 -8:00 // 06:00 14:00 -8:00 // // So from 02:00 - 02:59:59, we should return the daylight saving offset, instead of the standard offset. // internal static TimeSpan CalculateUtcOffset(DateTime time, DaylightTime daylightTimes) { if (daylightTimes==null) { return TimeSpan.Zero; } DateTimeKind kind = time.Kind; if (kind == DateTimeKind.Utc) { return TimeSpan.Zero; } DateTime startTime; DateTime endTime; // startTime and endTime represent the period from either the start of DST to the end and includes the // potentially overlapped times startTime = daylightTimes.Start + daylightTimes.Delta; endTime = daylightTimes.End; // For normal time zones, the ambiguous hour is the last hour of daylight saving when you wind the // clock back. It is theoretically possible to have a positive delta, (which would really be daylight // reduction time), where you would have to wind the clock back in the begnning. DateTime ambiguousStart; DateTime ambiguousEnd; if (daylightTimes.Delta.Ticks > 0) { ambiguousStart = endTime - daylightTimes.Delta; ambiguousEnd = endTime; } else { ambiguousStart = startTime; ambiguousEnd = startTime - daylightTimes.Delta; } Boolean isDst = false; if (startTime > endTime) { // In southern hemisphere, the daylight saving time starts later in the year, and ends in the beginning of next year. // Note, the summer in the southern hemisphere begins late in the year. if (time >= startTime || time < endTime) { isDst = true; } } else if (time>=startTime && time < endTime) { // In northern hemisphere, the daylight saving time starts in the middle of the year. isDst = true; } // If this date was previously converted from a UTC date and we were able to detect that the local // DateTime would be ambiguous, this data is stored in the DateTime to resolve this ambiguity. if (isDst && time >= ambiguousStart && time < ambiguousEnd) { isDst = time.IsAmbiguousDaylightSavingTime(); } if (isDst) { return daylightTimes.Delta; } return TimeSpan.Zero; } } }
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