Get the next date/time at which a daylight savings time transition occurs

Question

I would like to write (or use if it already exits) a function in C# that returns the date/time of the next DST transition given a System.TimeZoneInfo object and a particular "as of" time in that time zone. The time returned should be in the provided time zone. The function I want has this signature:

public DateTime GetNextTransition(DateTime asOfTime, TimeZoneInfo timeZone)
{
    // Implement me!
}

For example, if I pass in the "Eastern Standard Time" TimeZoneInfo object, and 1/21/2011@17:00 as the "asOfTime", I expect this function to return 3/13/2011@2:00.

The System.TimeZoneInfo.TransitionTime structure appears to have all the information I need, but ideally there would be some sort of built-in mechanism to convert the rule into an actual date. Anyone have any suggestions?

Answer 1

Take a look at the example on this page, I think it'll get what you need.

MSDN - TransitionTime

Answer 2

Hello_ there. It might be too late but I will post here the code that I used for this purpose. This could possibly safe someone's time to implement it. I did it actually with the help of the link is @Jamiegs answer.

    public static DateTime? GetNextTransition(DateTime asOfTime, TimeZoneInfo timeZone)
    {
        TimeZoneInfo.AdjustmentRule[] adjustments = timeZone.GetAdjustmentRules();
        if (adjustments.Length == 0)
        {
            // if no adjustment then no transition date exists
            return null;
        }

        int year = asOfTime.Year;
        TimeZoneInfo.AdjustmentRule adjustment = null;
        foreach (TimeZoneInfo.AdjustmentRule adj in adjustments)
        {
            // Determine if this adjustment rule covers year desired
            if (adj.DateStart.Year <= year && adj.DateEnd.Year >= year)
            {
                adjustment = adj;
                break;
            }
        }

        if (adjustment == null)
        {
            // no adjustment found so no transition date exists in the range
            return null;
        }


        DateTime dtAdjustmentStart = GetAdjustmentDate(adjustment.DaylightTransitionStart, year);
        DateTime dtAdjustmentEnd = GetAdjustmentDate(adjustment.DaylightTransitionEnd, year);


        if (dtAdjustmentStart >= asOfTime)
        {
            // if adjusment start date is greater than asOfTime date then this should be the next transition date
            return dtAdjustmentStart;
        }
        else if (dtAdjustmentEnd >= asOfTime)
        {
            // otherwise adjustment end date should be the next transition date
            return dtAdjustmentEnd;
        }
        else
        {
            // then it should be the next year's DaylightTransitionStart

            year++;
            foreach (TimeZoneInfo.AdjustmentRule adj in adjustments)
            {
                // Determine if this adjustment rule covers year desired
                if (adj.DateStart.Year <= year && adj.DateEnd.Year >= year)
                {
                    adjustment = adj;
                    break;
                }
            }

            dtAdjustmentStart = GetAdjustmentDate(adjustment.DaylightTransitionStart, year);
            return dtAdjustmentStart;
        }
    }


    public static DateTime GetAdjustmentDate(TimeZoneInfo.TransitionTime transitionTime, int year)
    {
        if (transitionTime.IsFixedDateRule)
        {
            return new DateTime(year, transitionTime.Month, transitionTime.Day);
        }
        else
        {
            // 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 = transitionTime.Week * 7 - 6;
            // What day of the week does the month start on?
            int firstDayOfWeek = (int)cal.GetDayOfWeek(new DateTime(year, transitionTime.Month, 1));
            // Determine how much start date has to be adjusted
            int transitionDay;
            int changeDayOfWeek = (int)transitionTime.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, transitionTime.Month))
                transitionDay -= 7;

            return new DateTime(year, transitionTime.Month, transitionDay, transitionTime.TimeOfDay.Hour, transitionTime.TimeOfDay.Minute, transitionTime.TimeOfDay.Second);
        }
    }

Sample usage will look like this:

// This should give you DateTime object for date 26 March 2017 
// because this date is first transition date after 1 January 2017 for Central Europe Standard Time zone
DateTime nextTransitionDate = GetNextTransition(new DateTime(2017, 1, 1), TimeZoneInfo.FindSystemTimeZoneById("Central Europe Standard Time"))

You can find the code that I played with here.