Formatting local timestamps with WinRT - datetime

Question

WinRT uses the DateTimeFormatter class to turn timestamps into human-readable dates. In C++CX, you'll pass it a DateTime instance, which contains a timestamp in UTC time, and let it work its magic.
However, I have an application that consumes timestamps in local time. I'd like to format them and show them to my users, but if I pass the timestamp as is, the DateTimeFormatter will assume that it's UTC and will try to convert it to local time again, resulting in incorrect times.
How can I display local time with WinRT? Is there a way to turn back local time into UTC time?
The timestamps are generated from the machine that consumes them, so there is no risk of timezone confusion. It would also be technically feasible to produce UTC timestamps instead, but this would be rather inconvenient and I'd like to fall back to that only if it's the only way.

Answer 1

Thankfully, FileTimeToSystemTime, TzSpecificLocalTimeToSystemTime and SystemTimeToFileTime are all available to Windows store apps. With that, it's possible to create a function to change local back to UTC.
uint64 LocalTimeToUtcTime(uint64 local)
{
LARGE_INTEGER largeTime;
largeTime.QuadPart = local;
FILETIME intermediate;
intermediate.dwHighDateTime = largeTime.HighPart;
intermediate.dwLowDateTime = largeTime.LowPart;
SYSTEMTIME systemLocal, systemUtc;
if (!FileTimeToSystemTime(&intermediate, &systemLocal))
{
// handle error
}
if (!TzSpecificLocalTimeToSystemTime(nullptr, &systemLocal, &systemUtc))
{
// handle error
}
if (!SystemTimeToFileTime(&systemUtc, &intermediate))
{
// handle error
}
largeTime.HighPart = intermediate.dwHighDateTime;
largeTime.LowPart = intermediate.dwLowDateTime;
return largeTime.QuadPart;
}

Answer 2

You can use the Windows::Globalization::Calendar class to work with local time, or with time in any time zone.
The Calendar defaults to the local time zone if you don't explicitly set one. You can then use GetDateTime() to retrieve a Windows::Foundation::DateTime instance that can be used with DateTimeFormatter.
Calendar^ cal = ref new Calendar();
cal->SetToMin();
cal->Year = 2014;
cal->Month = 7;
cal->Day = 14;
cal->Hour = 12;
cal->Minute = 34;
cal->Second = 56;
DateTime dt = cal->GetDateTime();
DateTimeFormatter^ dtf = ref new DateTimeFormatter("shortdate shorttime");
String^ result = dtf->Format(dt);
Logger::WriteMessage(result->Data());