TimeZoneInfo.TransitionTime.IsFixedDateRule Property

public:
 property bool IsFixedDateRule { bool get(); };

public bool IsFixedDateRule { get; }

member this.IsFixedDateRule : bool

Public ReadOnly Property IsFixedDateRule As Boolean

Property Value

true if the time change rule is fixed-date; false if the time change rule is floating-date.

Examples

The following example lists the transition times to and from daylight saving time for all time zones that are available on the local system. For time zones with fixed-date rules, it displays transition time information from the properties of the TimeZoneInfo.TransitionTime object. For time zones without fixed-date rules, it uses a Calendar object that represents the current system calendar to determine the actual start and end dates of the transition. The example displays the results to the console.

private void GetTransitionTimes(int year)
{
   // Instantiate DateTimeFormatInfo object for month names
   DateTimeFormatInfo dateFormat = CultureInfo.CurrentCulture.DateTimeFormat;

   // Get and iterate time zones on local computer
   ReadOnlyCollection<TimeZoneInfo> timeZones = TimeZoneInfo.GetSystemTimeZones();
   foreach (TimeZoneInfo timeZone in timeZones)
   {
      Console.WriteLine("{0}:", timeZone.StandardName);
      TimeZoneInfo.AdjustmentRule[] adjustments = timeZone.GetAdjustmentRules();
      int startYear = year;
      int endYear = startYear;

      if (adjustments.Length == 0)
      {
         Console.WriteLine("   No adjustment rules.");
      }   
      else
      {   
         TimeZoneInfo.AdjustmentRule adjustment = GetAdjustment(adjustments, year);
         if (adjustment == null)
         {
            Console.WriteLine("   No adjustment rules available for this year.");
            continue;
         }
         TimeZoneInfo.TransitionTime startTransition, endTransition;
         
         // Determine if starting transition is fixed 
         startTransition = adjustment.DaylightTransitionStart;
         // Determine if starting transition is fixed and display transition info for year
         if (startTransition.IsFixedDateRule)
            Console.WriteLine("   Begins on {0} {1} at {2:t}", 
                              dateFormat.GetMonthName(startTransition.Month), 
                              startTransition.Day, 
                              startTransition.TimeOfDay);
         else
            DisplayTransitionInfo(startTransition, startYear, "Begins on");

         // Determine if ending transition is fixed and display transition info for year
         endTransition = adjustment.DaylightTransitionEnd;
         
         // Does the transition back occur in an earlier month (i.e., 
         // the following year) than the transition to DST? If so, make
         // sure we have the right adjustment rule.
         if (endTransition.Month < startTransition.Month)
         {
            endTransition = GetAdjustment(adjustments, year + 1).DaylightTransitionEnd;
            endYear++;
         }
      
         if (endTransition.IsFixedDateRule)
            Console.WriteLine("   Ends on {0} {1} at {2:t}", 
                              dateFormat.GetMonthName(endTransition.Month), 
                              endTransition.Day, 
                              endTransition.TimeOfDay);
         else
            DisplayTransitionInfo(endTransition, endYear, "Ends on");
      }      
   }   
} 

private static TimeZoneInfo.AdjustmentRule GetAdjustment(TimeZoneInfo.AdjustmentRule[] adjustments,
                                                  int year)
{                                                  
   // Iterate adjustment rules for time zone
   foreach (TimeZoneInfo.AdjustmentRule adjustment in adjustments)
   {
      // Determine if this adjustment rule covers year desired
      if (adjustment.DateStart.Year <= year && adjustment.DateEnd.Year >= year)
         return adjustment;
   }   
   return null;
}

private void DisplayTransitionInfo(TimeZoneInfo.TransitionTime transition, int year, string label)
{
   // For non-fixed date rules, get local calendar
   Calendar cal = CultureInfo.CurrentCulture.Calendar;
   // Get first day of week for transition
   // For example, the 3rd week starts no earlier than the 15th of the month
   int startOfWeek = transition.Week * 7 - 6;
   // What day of the week does the month start on?
   int firstDayOfWeek = (int) cal.GetDayOfWeek(new DateTime(year, transition.Month, 1)); 
   // Determine how much start date has to be adjusted
   int transitionDay; 
   int changeDayOfWeek = (int) transition.DayOfWeek;

   if (firstDayOfWeek <= changeDayOfWeek)
      transitionDay = startOfWeek + (changeDayOfWeek - firstDayOfWeek);
   else     
      transitionDay = startOfWeek + (7 - firstDayOfWeek + changeDayOfWeek);

   // Adjust for months with no fifth week
   if (transitionDay > cal.GetDaysInMonth(year, transition.Month))  
      transitionDay -= 7;

   Console.WriteLine("   {0} {1}, {2:d} at {3:t}", 
                     label, 
                     transition.DayOfWeek, 
                     new DateTime(year, transition.Month, transitionDay), 
                     transition.TimeOfDay);   
}

open System
open System.Globalization

let displayTransitionInfo (transition: TimeZoneInfo.TransitionTime) year label =
    // For non-fixed date rules, get local calendar
    let cal = CultureInfo.CurrentCulture.Calendar
    // Get first day of week for transition
    // For example, the 3rd week starts no earlier than the 15th of the month
    let startOfWeek = transition.Week * 7 - 6
    // What day of the week does the month start on?
    let firstDayOfWeek = cal.GetDayOfWeek(DateTime(year, transition.Month, 1)) |> int 
    // Determine how much start date has to be adjusted
    let changeDayOfWeek = int transition.DayOfWeek

    let transitionDay = 
        if firstDayOfWeek <= changeDayOfWeek then
            startOfWeek + (changeDayOfWeek - firstDayOfWeek)
        else     
            startOfWeek + (7 - firstDayOfWeek + changeDayOfWeek)

    // Adjust for months with no fifth week
    let transitionDay =
        if transitionDay > cal.GetDaysInMonth(year, transition.Month) then  
            transitionDay - 7
        else 
            transitionDay

    printfn $"   {label} {transition.DayOfWeek}, {DateTime(year, transition.Month, transitionDay):d} at {transition.TimeOfDay:t}"   

let getAdjustment (adjustments: TimeZoneInfo.AdjustmentRule seq) year =
    adjustments
    // Iterate adjustment rules for time zone
    // Determine if this adjustment rule covers year desired
    |> Seq.tryFind (fun adjustment -> adjustment.DateStart.Year <= year && adjustment.DateEnd >= DateTime year)
    |> Option.defaultValue null

let getTransitionTimes year =
    // Instantiate DateTimeFormatInfo object for month names
    let dateFormat = CultureInfo.CurrentCulture.DateTimeFormat

    // Get and iterate time zones on local computer
    let timeZones = TimeZoneInfo.GetSystemTimeZones()
    for timeZone in timeZones do
        printfn $"{timeZone.StandardName}:"
        let adjustments = timeZone.GetAdjustmentRules()
        let startYear = year
        let mutable endYear = startYear

        if adjustments.Length = 0 then
            printfn "   No adjustment rules."
        else
            let adjustment = getAdjustment adjustments year
            if adjustment = null then
                Console.WriteLine("   No adjustment rules available for this year.")
            else
                // Determine if starting transition is fixed 
                let startTransition = adjustment.DaylightTransitionStart
                // Determine if starting transition is fixed and display transition info for year
                if startTransition.IsFixedDateRule then
                    printfn $"   Begins on {dateFormat.GetMonthName startTransition.Month} {startTransition.Day} at {startTransition.TimeOfDay:t}"
                else
                    displayTransitionInfo startTransition startYear "Begins on"

                // Determine if ending transition is fixed and display transition info for year
                let mutable endTransition = adjustment.DaylightTransitionEnd
                
                // Does the transition back occur in an earlier month (i.e., 
                // the following year) than the transition to DST? If so, make
                // sure we have the right adjustment rule.
                if endTransition.Month < startTransition.Month then
                    endTransition <- (getAdjustment adjustments (year + 1)).DaylightTransitionEnd
                    endYear <- endYear + 1
                
                if endTransition.IsFixedDateRule then
                    printfn $"   Ends on {dateFormat.GetMonthName endTransition.Month} {endTransition.Day} at {endTransition.TimeOfDay:t}"
                else
                    displayTransitionInfo endTransition endYear "Ends on"

Private Sub GetTransitionTimes(year As Integer)
   ' Get and iterate time zones on local computer
   Dim timeZones As ReadOnlyCollection(Of TimeZoneInfo) = TimeZoneInfo.GetSystemTimeZones()
   For Each timeZone As TimeZoneInfo In timeZones
      Console.WriteLine("{0}:", timeZone.StandardName)
      Dim adjustments() As TimeZoneInfo.AdjustmentRule = timeZone.GetAdjustmentRules()
      Dim startYear As Integer = year
      Dim endYear As Integer = startYear

      If adjustments.Length = 0 Then
         Console.WriteLine("   No adjustment rules.")
      Else   
         Dim adjustment As TimeZoneInfo.AdjustmentRule = GetAdjustment(adjustments, year)
         If adjustment Is Nothing Then
            Console.WriteLine("   No adjustment rules available for this year.")
            Continue For
         End If
         Dim startTransition, endTransition As TimeZoneInfo.TransitionTime

         ' Determine if starting transition is fixed 
         startTransition = adjustment.DaylightTransitionStart
         ' Determine if starting transition is fixed and display transition info for year
         If startTransition.IsFixedDateRule Then
            Console.WriteLine("   Begins on {0} {1} at {2:t}", _
                              MonthName(startTransition.Month), _
                              startTransition.Day, _
                              startTransition.TimeOfDay)
         Else
            DisplayTransitionInfo(startTransition, startYear, "Begins on")
         End If    

         ' Determine if ending transition is fixed and display transition info for year
         endTransition = adjustment.DaylightTransitionEnd
                
         ' Does the transition back occur in an earlier month (i.e., 
         ' the following year) than the transition to DST? If so, make
         ' sure we have the right adjustment rule.
         If endTransition.Month < startTransition.Month Then
            endTransition = GetAdjustment(adjustments, year + 1).DaylightTransitionEnd
            endYear += 1
         End If

         If endTransition.IsFixedDateRule Then
            Console.WriteLine("   Ends on {0} {1} at {2:t}", _
                              MonthName(endTransition.Month), _
                              endTransition.Day, _
                              endTransition.TimeOfDay)
         Else
            DisplayTransitionInfo(endTransition, endYear, "Ends on")
         End If    
      End If   
   Next 
End Sub

Private Function GetAdjustment(adjustments As TimeZoneInfo.AdjustmentRule(), _
                               year As Integer) As TimeZoneInfo.AdjustmentRule
   ' Iterate adjustment rules for time zone
   For Each adjustment As TimeZoneInfo.AdjustmentRule In adjustments
      ' Determine if this adjustment rule covers year desired
      If adjustment.DateStart.Year <= year And adjustment.DateEnd.Year >= year Then
         Return adjustment
      End If
   Next   
   Return Nothing
End Function
   
Private Sub DisplayTransitionInfo(transition As TimeZoneInfo.TransitionTime, year As Integer, label As String)
   ' For non-fixed date rules, get local calendar
   Static cal As Calendar = CultureInfo.CurrentCulture.Calendar

   ' Get first day of week for transition
   ' For example, the 3rd week starts no earlier than the 15th of the month
   Dim startOfWeek As Integer = transition.Week * 7 - 6
   ' What day of the week does the month start on?
   Dim firstDayOfWeek As Integer = cal.GetDayOfWeek(New Date(year, transition.Month, 1))
   ' Determine how much start date has to be adjusted
   Dim transitionDay As Integer 
   Dim changeDayOfWeek As Integer = transition.DayOfWeek

   If firstDayOfWeek <= changeDayOfWeek Then
      transitionDay = startOfWeek + (changeDayOfWeek - firstDayOfWeek)
   Else     
      transitionDay = startOfWeek + (7 - firstDayOfWeek + changeDayOfWeek)
   End If
   ' Adjust for months with no fifth week
   If transitionDay > cal.GetDaysInMonth(year, transition.Month) Then  
      transitionDay -= 7
   End If
   Console.WriteLine("   {0} {1}, {2:d} at {3:t}", _
                     label, _
                     transition.DayOfWeek, _
                     New DateTime(year, transition.Month, transitionDay), _
                     transition.TimeOfDay)   
End Sub

Remarks

A fixed-date rule indicates that the transition occurs on the same date and time of each year to which the adjustment rule applies. For example, a time change that occurs every November 3 follows a fixed-date rule. A floating-date rule indicates that the transition occurs on a specific day of a specific week of a specific month for each year to which the adjustment rule applies. For example, a time change that occurs on the first Sunday of November follows a floating-date rule.

The value of the IsFixedDateRule property determines which properties of a TimeZoneInfo.TransitionTime object have valid values. The following table indicates which properties are affected by the value of the IsFixedDateRule property.

Applies to