FILETIME to/from ISO 8601 with Win32 API. Getting DST right?

Question

Wrote something in .NET; it works well. Now I am trying to rewrite it as a shell extension with the Win32 API. Ultimately I want to convert FILETIMEs to and from ISO-8601 strings. This is doable without fuss, using GetTimeZoneInformation and FileTimeToSystemTime and SystemTimeToTzSpecificLocalTime and StringCchPrintf to assemble the members of the SYSTEMTIME and TIME_ZONE_INFORMATION structs into a string.

The problem, as usual when working with date/times, is Daylight Saving Time. Using GetTimeZoneInformation tells me the UTC offset that's in effect now. Using .NET's DateTime.ToString("o") takes into account the daylight saving time at the time represented in the DateTime.

Example for the same FILETIME:
Output of ToString("o"): 2017-06-21T12:00:00.0000000-05:00
Output of chained APIs: 2017-06-21T12:00:00-06:00

The UTC offset is wrong coming from the chained API calls. How does .NET's DateTime do it? How do we replicate that in Win32? Is there something like GetTimeZoneInformationForYear, but instead of for a year, for a moment in local time?

Answer 1

First, I use DYNAMIC_TIME_ZONE_INFORMATION structure and GetDynamicTimeZoneInformation

DYNAMIC_TIME_ZONE_INFORMATION and TIME_ZONE_INFORMATION also has a DaylightBias member:

The bias value to be used during local time translations that occur during daylight saving time. This member is ignored if a value for the DaylightDate member is not supplied.

This value is added to the value of the Bias member to form the bias used during daylight saving time. In most time zones, the value of this member is –60.

So, if the date is in daylight saving time, you need to add this DaylightBias to Bias. In addition, you can determine whether the current date is daylight saving time according to the description in DaylightDate:

To select the correct day in the month, set the wYear member to zero, the wHour and wMinute members to the transition time, the wDayOfWeek member to the appropriate weekday, and the wDay member to indicate the occurrence of the day of the week within the month (1 to 5, where 5 indicates the final occurrence during the month if that day of the week does not occur 5 times).

If the wYear member is not zero, the transition date is absolute; it will only occur one time. Otherwise, it is a relative date that occurs yearly.