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c++winapitimezonedst

How to find out if the current local time is in daylight savings

GetTimeZoneInformation fills up a TIME_ZONE_INFO struct with all sorts of useful information.

Except it doesn't actually tell me if the current local time zone is in daylight savings or not.

For that matter, I'm in New York and the StandardBias and DaylightBias both say the same thing as the Bias.

What's going on?

Solution

  • The return code of GetTimeZoneInformation contains the information you want.

    According to MSDN:

    If the function succeeds, it returns one of the following values.

    TIME_ZONE_ID_UNKNOWN
    0
    Daylight saving time is not used in the current time zone, because there are no transition dates or automatic adjustment for daylight saving time is disabled.

    TIME_ZONE_ID_STANDARD
    1
    The system is operating in the range covered by the StandardDate member of the TIME_ZONE_INFORMATION structure.

    TIME_ZONE_ID_DAYLIGHT
    2
    The system is operating in the range covered by the DaylightDate member of the TIME_ZONE_INFORMATION structure.

    If the function fails for other reasons, such as an out of memory error, it returns TIME_ZONE_ID_INVALID. To get extended error information, call GetLastError.

    Make sure you check the return code to ensure the information you are receiving is indeed valid.