# How to: Create time zones with adjustment rules

The precise time zone information that is required by an application may not be present on a particular system for several reasons:

• The time zone has never been defined in the local system's registry.

• Data about the time zone has been modified or removed from the registry.

• The time zone does not have accurate information about time zone adjustments for a particular historic period.

In these cases, you can call the CreateCustomTimeZone method to define the time zone required by your application. You can use the overloads of this method to create a time zone with or without adjustment rules. If the time zone supports daylight saving time, you can define adjustments with either fixed or floating adjustment rules. (For definitions of these terms, see the "Time Zone Terminology" section in Time zone overview.)

Important

Custom time zones created by calling the CreateCustomTimeZone method are not added to the registry. Instead, they can be accessed only through the object reference returned by the CreateCustomTimeZone method call.

This topic shows how to create a time zone with adjustment rules. To create a time zone that does not support daylight saving time adjustment rules, see How to: Create Time Zones Without Adjustment Rules.

### To create a time zone with floating adjustment rules

1. For each adjustment (that is, for each transition away from and back to standard time over a particular time interval), do the following:

1. Define the starting transition time for the time zone adjustment.

You must call the TimeZoneInfo.TransitionTime.CreateFloatingDateRule method and pass it a DateTime value that defines the time of the transition, an integer value that defines the month of the transition, an integer value that defines the week on which the transition occurs, and a DayOfWeek value that defines the day of the week on which the transition occurs. This method call instantiates a TimeZoneInfo.TransitionTime object.

2. Define the ending transition time for the time zone adjustment. This requires another call to the TimeZoneInfo.TransitionTime.CreateFloatingDateRule method. This method call instantiates a second TimeZoneInfo.TransitionTime object.

3. Call the CreateAdjustmentRule method and pass it the effective start and end dates of the adjustment, a TimeSpan object that defines the amount of time in the transition, and the two TimeZoneInfo.TransitionTime objects that define when the transitions to and from daylight saving time occur. This method call instantiates a TimeZoneInfo.AdjustmentRule object.

2. Define the time zone's display name. The display name follows a fairly standard format in which the time zone's offset from Coordinated Universal Time (UTC) is enclosed in parentheses and is followed by a string that identifies the time zone, one or more of the cities in the time zone, or one or more of the countries or regions in the time zone.

3. Define the name of the time zone's standard time. Typically, this string is also used as the time zone's identifier.

4. Define the name of the time zone's daylight time.

5. If you want to use a different identifier than the time zone's standard name, define the time zone identifier.

6. Instantiate a TimeSpan object that defines the time zone's offset from UTC. Time zones with times that are later than UTC have a positive offset. Time zones with times that are earlier than UTC have a negative offset.

7. Call the TimeZoneInfo.CreateCustomTimeZone(String, TimeSpan, String, String, String, TimeZoneInfo+AdjustmentRule[]) method to instantiate the new time zone.

## Example

The following example defines a Central Standard Time zone for the United States that includes adjustment rules for a variety of time intervals from 1918 to the present.

TimeZoneInfo cst;
// Declare necessary TimeZoneInfo.AdjustmentRule objects for time zone
TimeSpan delta = new TimeSpan(1, 0, 0);
// Declare transition time variables to hold transition time information
TimeZoneInfo.TransitionTime transitionRuleStart, transitionRuleEnd;

// Define new Central Standard Time zone 6 hours earlier than UTC
// Define rule 1 (for 1918-1919)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 03, 05, DayOfWeek.Sunday);
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 10, 05, DayOfWeek.Sunday);
transitionRuleStart, transitionRuleEnd);
// Define rule 2 (for 1942)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(new DateTime(1, 1, 1, 2, 0, 0), 02, 09);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 3 (for 1945)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(new DateTime(1, 1, 1, 23, 0, 0), 08, 14);
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFixedDateRule(new DateTime(1, 1, 1, 2, 0, 0), 09, 30);
delta, transitionRuleStart, transitionRuleEnd);
// Define end rule (for 1967-2006)
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 10, 5, DayOfWeek.Sunday);
// Define rule 4 (for 1967-73)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 04, 05, DayOfWeek.Sunday);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 5 (for 1974 only)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(new DateTime(1, 1, 1, 2, 0, 0), 01, 06);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 6 (for 1975 only)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(new DateTime(1, 1, 1, 2, 0, 0), 02, 23);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 7 (1976-1986)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 04, 05, DayOfWeek.Sunday);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 8 (1987-2006)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 04, 01, DayOfWeek.Sunday);
delta, transitionRuleStart, transitionRuleEnd);
// Define rule 9 (2007- )
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 03, 02, DayOfWeek.Sunday);
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(new DateTime(1, 1, 1, 2, 0, 0), 11, 01, DayOfWeek.Sunday);
delta, transitionRuleStart, transitionRuleEnd);

// Convert list of adjustment rules to an array

cst = TimeZoneInfo.CreateCustomTimeZone("Central Standard Time", new TimeSpan(-6, 0, 0),
"(GMT-06:00) Central Time (US Only)", "Central Standard Time",

Dim cst As TimeZoneInfo
' Declare necessary TimeZoneInfo.AdjustmentRule objects for time zone
Dim delta As New TimeSpan(1, 0, 0)
' Declare transition time variables to hold transition time information
Dim transitionRuleStart, transitionRuleEnd As TimeZoneInfo.TransitionTime

' Define new Central Standard Time zone 6 hours earlier than UTC
' Define rule 1 (for 1918-1919)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#02:00:00AM#, 03, 05, DayOfWeek.Sunday)
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#02:00:00AM#, 10, 05, DayOfWeek.Sunday)
' Define rule 2 (for 1942)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(#2:00:00AM#, 02, 09)
' Define rule 3 (for 1945)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(#11:00:00PM#, 08, 14)
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFixedDateRule(#2:00:00AM#, 09, 30)
' Define end rule (for 1967-2006)
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#02:00:00AM#, 10, 5, DayOfWeek.Sunday)
' Define rule 4 (for 1967-73)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#2:00:00AM#, 04, 05, DayOfWeek.Sunday)
' Define rule 5 (for 1974 only)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(#2:00:00AM#, 01, 06)
' Define rule 6 (for 1975 only)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFixedDateRule(#2:00:00AM#, 02, 23)
' Define rule 7 (1976-1986)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#2:00:00AM#, 04, 05, DayOfWeek.Sunday)
' Define rule 8 (1987-2006)
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#2:00:00AM#, 04, 01, DayOfWeek.Sunday)
' Define rule 9 (2007- )
transitionRuleStart = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#2:00:00AM#, 03, 02, DayOfWeek.Sunday)
transitionRuleEnd = TimeZoneInfo.TransitionTime.CreateFloatingDateRule(#2:00:00AM#, 11, 01, DayOfWeek.Sunday)

' Convert list of adjustment rules to an array

cst = TimeZoneInfo.CreateCustomTimeZone("Central Standard Time", New TimeSpan(-6, 0, 0), _
"(GMT-06:00) Central Time (US Only)", "Central Standard Time", _


The time zone created in this example has multiple adjustment rules. Care must be taken to ensure that the effective start and end dates of any adjustment rule do not overlap with the dates of another adjustment rule. If there is an overlap, an InvalidTimeZoneException is thrown.

For floating adjustment rules, the value 5 is passed to the week parameter of the CreateFloatingDateRule method to indicate that the transition occurs on the last week of a particular month.

In creating the array of TimeZoneInfo.AdjustmentRule objects to use in the TimeZoneInfo.CreateCustomTimeZone(String, TimeSpan, String, String, String, TimeZoneInfo+AdjustmentRule[]) method call, the code could initialize the array to the size required by the number of adjustments to be created for the time zone. Instead, this code example calls the Add method to add each adjustment rule to a generic List<T> collection of TimeZoneInfo.AdjustmentRule objects. The code then calls the CopyTo method to copy the members of this collection to the array.

The example also uses the CreateFixedDateRule method to define fixed-date adjustments. This is similar to calling the CreateFloatingDateRule method, except that it requires only the time, month, and day of the transition parameters.

The example can be tested using code such as the following:

TimeZoneInfo est = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

DateTime pastDate1 = new DateTime(1942, 2, 11);
Console.WriteLine("Is {0} daylight saving time: {1}", pastDate1,
cst.IsDaylightSavingTime(pastDate1));

DateTime pastDate2 = new DateTime(1967, 10, 29, 1, 30, 00);
Console.WriteLine("Is {0} ambiguous: {1}", pastDate2,
cst.IsAmbiguousTime(pastDate2));

DateTime pastDate3 = new DateTime(1974, 1, 7, 2, 59, 00);
Console.WriteLine("{0} {1} is {2} {3}", pastDate3,
est.IsDaylightSavingTime(pastDate3) ?
est.DaylightName : est.StandardName,
TimeZoneInfo.ConvertTime(pastDate3, est, cst),
cst.IsDaylightSavingTime(TimeZoneInfo.ConvertTime(pastDate3, est, cst)) ?
cst.DaylightName : cst.StandardName);
//
// This code produces the following output to the console:
//
//    Is 2/11/1942 12:00:00 AM daylight saving time: True
//    Is 10/29/1967 1:30:00 AM ambiguous: True
//    1/7/1974 2:59:00 AM Eastern Standard Time is 1/7/1974 2:59:00 AM Central Daylight Time

Dim est As TimeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time")

Dim pastDate1 As Date = #2/11/1942#
Console.WriteLine("Is {0} daylight saving time: {1}", pastDate1, _
cst.IsDaylightSavingTime(pastDate1))

Dim pastDate2 As Date = #10/29/1967 1:30AM#
Console.WriteLine("Is {0} ambiguous: {1}", pastDate2, _
cst.IsAmbiguousTime(pastDate2))

Dim pastDate3 As Date = #1/7/1974 2:59AM#
Console.WriteLine("{0} {1} is {2} {3}", pastDate3, _
IIf(est.IsDaylightSavingTime(pastDate3), _
est.DaylightName, est.StandardName), _
TimeZoneInfo.ConvertTime(pastDate3, est, cst), _
IIf(cst.IsDaylightSavingTime(TimeZoneInfo.ConvertTime(pastDate3, est, cst)), _
cst.DaylightName, cst.StandardName))
'
' This code produces the following output to the console:
'
'    Is 2/11/1942 12:00:00 AM daylight saving time: True
'    Is 10/29/1967 1:30:00 AM ambiguous: True
'    1/7/1974 2:59:00 AM Eastern Standard Time is 1/7/1974 2:59:00 AM Central Daylight Time


## Compiling the code

This example requires:

• That the following namespaces be imported:

using System.Collections.Generic;
using System.Collections.ObjectModel;

Imports System.Collections.Generic
Imports System.Collections.ObjectModel