How can I add a day to a DateTime in a specific timezone

Question

Is it possible to (reasonably easy) add a day to a date in another timezone than that of DateTime.Local, while respecting the different adjustment rules (DST etc) for that particular timezone?

var rst = TimeZoneInfo.FindSystemTimeZoneById("Romance Standard Time");
var dayInSpecificTimezone = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, rst); // Got the datetime in specified timezone
// now I would like to add a "day" to that, still observing the rules of that timezone.. something like:
var sameTimeTheDayAfterThat = dayInSpecificTimezone.AddDays(1, rst); // no such method exists

Answer 1

Here are extension methods that you can use for this.

First, this AddDays method matches the signature you were asking about. It operates on DateTime values:

public static DateTime AddDays(this DateTime dt, double days, TimeZoneInfo tz)
{
    // If the kind is Local or Utc, convert that point in time to the given time zone
    DateTimeKind originalKind = dt.Kind;
    if (originalKind != DateTimeKind.Unspecified)
    {
        dt = TimeZoneInfo.ConvertTime(dt, tz);
    }

    // Add days with respect to the wall time only
    DateTime added = dt.AddDays(days);

    // Resolve the added value to a specific point in time
    DateTimeOffset resolved = added.ToDateTimeOffset(tz);

    // Return only the DateTime portion, but take the original kind into account
    switch (originalKind)
    {
        case DateTimeKind.Local:
            return resolved.LocalDateTime;
        case DateTimeKind.Utc:
            return resolved.UtcDateTime;
        default: // DateTimeKind.Unspecified
            return resolved.DateTime;
    }
}

Here is another variation of that extension method. This one operates on DateTimeOffset values:

public static DateTimeOffset AddDays(this DateTimeOffset dto, double days, TimeZoneInfo tz)
{
    // Make sure the input time is in the provided time zone
    dto = TimeZoneInfo.ConvertTime(dto, tz);

    // Add days with respect to the wall time only
    DateTime added = dto.DateTime.AddDays(days);

    // Resolve the added value to a specific point in time
    DateTimeOffset resolved = added.ToDateTimeOffset(tz);

    // Return the fully resolved value
    return resolved;
}

Both of the above methods depend on the following ToDateTimeOffset extension method (which I've used in a few different posts now).

public static DateTimeOffset ToDateTimeOffset(this DateTime dt, TimeZoneInfo tz)
{
    if (dt.Kind != DateTimeKind.Unspecified)
    {
        // Handle UTC or Local kinds (regular and hidden 4th kind)
        DateTimeOffset dto = new DateTimeOffset(dt.ToUniversalTime(), TimeSpan.Zero);
        return TimeZoneInfo.ConvertTime(dto, tz);
    }

    if (tz.IsAmbiguousTime(dt))
    {
        // Prefer the daylight offset, because it comes first sequentially (1:30 ET becomes 1:30 EDT)
        TimeSpan[] offsets = tz.GetAmbiguousTimeOffsets(dt);
        TimeSpan offset = offsets[0] > offsets[1] ? offsets[0] : offsets[1];
        return new DateTimeOffset(dt, offset);
    }

    if (tz.IsInvalidTime(dt))
    {
        // Advance by the gap, and return with the daylight offset  (2:30 ET becomes 3:30 EDT)
        TimeSpan[] offsets = { tz.GetUtcOffset(dt.AddDays(-1)), tz.GetUtcOffset(dt.AddDays(1)) };
        TimeSpan gap = offsets[1] - offsets[0];
        return new DateTimeOffset(dt.Add(gap), offsets[1]);
    }

    // Simple case
    return new DateTimeOffset(dt, tz.GetUtcOffset(dt));
}

Lastly, I'll point out that there is another option to consider: Use the Noda Time library. It's ZoneDateTime.Add method has exactly this purpose.

Answer 2

Adding a day to a DateTime object and displaying the date in a particular Time Zone are two separate things.

The DateTime.AddDays function can be used to add the day (i.e. Add 24 hours to your current variable). You can then display that date time in any Time Zone you prefer.

For example:

var rst = TimeZoneInfo.FindSystemTimeZoneById("Romance Standard Time");
var dayInSpecificTimezone = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, rst); // Got the datetime in specified timezone

Console.WriteLine("RST datetime now is {0}", dayInSpecificTimezone);

var sameTimeTheDayAfterThat = dayInSpecificTimezone.AddDays(1);

Console.WriteLine("RST datetime in 1 day is {0}", sameTimeTheDayAfterThat);
Console.WriteLine("local datetime in 1 day is {0}", TimeZoneInfo.ConvertTime(sameTimeTheDayAfterThat, rst, TimeZoneInfo.Local));

would give output similar to:

RST datetime now is 29/01/2020 4:31:14 AM
RST datetime in 1 day is 30/01/2020 4:31:14 AM
local datetime in 1 day is 30/01/2020 1:31:14 PM