TimeZone.AtStartofDate for day with switch to DLS in Brazil

[Stoffelche]

In my code I have the following method:

public DateTime GetUtcStartOfDay(DateTime day) {
 return TimeZone.AtStartOfDay(new NodaTime.LocalDate(day.Year, day.Month, day.Day)).ToDateTimeUtc();
}

for 2012/10/21 I would assume that it returns UTC 2012-10-21-03:00 as the time is advanced in Brazil 1 hour but nothing happened yet with respect to UTC.

Then for the following day 2012/10/22 I would assume to get UTC 2012-10-22-02:00 as the missing hour leads to the shorter day 21.

However for 2012/10/21 I get instead: 2012-10-21-02:59:59.999, which is 1 ms less then expected.
This millisecond is already missing iin the AtStartOfDay result, so I assume something is wrong here?
For the following day 22 I get the full hour as expected.

Same applies for all other years for the day where the 1 hour is advanced.

BTW I use BclDateTimeZone.

[jskeet]

BTW I use BclDateTimeZone.

I suspect that's the problem. TimeZoneInfo behaves weirdly in all kinds of ways. I'll look into it - but I strongly suspect that if you use the TZDB data instead, it'll work fine.

[jskeet]

Complete example:

using System;
using NodaTime;
using NodaTime.Text;

class Test
{
    static void Main()
    {
        var zone = DateTimeZoneProviders.Bcl["E. South America Standard Time"];
        var date = new LocalDate(2012, 10, 21);
        var zoned = zone.AtStartOfDay(date);
        Console.WriteLine(ZonedDateTimePattern.ExtendedFormatOnlyIso.Format(zoned));
    }
}

[jskeet]

I suspect the problem is that TimeZoneInfo uses "23:59:59.999" to represent a transition at midnight of the following day. I'll need to investigate whether our behavior actually matches TimeZoneInfo, or whether we just need to "round up".

[Stoffelche]

for now I patched it the following way:

public DateTime GetUtcStartOfDay(DateTime day) {
	DateTime dt = TimeZone.AtStartOfDay(new NodaTime.LocalDate(day.Year, day.Month, day.Day)).ToDateTimeUtc();
// for Brazil on day of DLS switch TimeZon.AtStartOfDay looses a millisecond
	long seconds = dt.Ticks / TimeSpan.TicksPerSecond;
	if (seconds * TimeSpan.TicksPerSecond < dt.Ticks)
		dt = new DateTime((seconds + 1) * TimeSpan.TicksPerSecond, DateTimeKind.Utc);
	return dt;
}

[jskeet]

Thanks very much for raising this. It's fixed in master, and will go out as part of 3.0 (or the next beta). I won't be backporting it to 2.x.

[Stoffelche]

Thanks for fixing. When will v3.0 be released?