Convert datetime from one timezone to another (native C++)

Customers from around the world can send certain 'requests' to my server application. All these customers are located in many different time zones.

For every request, I need to map the request to an internal C++ class instance. Every class instance has some information about its 'location', which is also indicated by a time zone.

Every customer can send requests relating to instances belonging to different time zones. To prevent my customers from开发者_如何转开发 converting everything themselves to the time zone of the 'target' instance, I have to convert everything myself from one time zone to another. However, I only find in C++ (unmanaged, native) functions to convert times between local time and GTM, but not from/to a time zone that is not your current time zone.

I could ask my customers to send every date time in UTC or GTM, but that does not solve my problem as I still have to convert this to the time zone of the 'instance', which can be any time zone in the world.

I also don't seem to find a Windows function that does this. What I do find is a managed .Net class that does this, but I want to keep my application strictly unmanaged.

Are there any Windows (XP, Vista, 7, 2003, 2008) functions that I can use (and which I overlooked in the documentation), or are there any other free algorithms that can convert between one time zone and the other?

Notice that it is not the GMT-difference that is posing the problem, but the actual DST-transition moment that seems to depend on the time zone. E.g:

• Western Europe goes from non-DST to DST the last Sunday before April 1st.
• USA goes from non-DST to DST the 2nd Sunday after March 1st.
• China has no DST.
• Australia goes from non-DST to DST the 1st Sunday after October 1st.

All this DST-transition information is available somewhere in the Windows registry. Problem is: which Windows function can I use to exploit this information.

I don't know of a way to extract information about other TimeZones via the API: I've seen it done by querying the registry though (we do this in a WindowsCE-based product).

The TimeZones are defined as registry keys under

HKLM\Software\Microsoft\Windows NT\Current Version\Time Zones

Each key contains several values, and the one which tells you about offsets & Daylight Savings is the TZI key. This is a binary blob, and it represents this structure:

typedef struct
{
    LONG m_nBias;
    LONG m_nStandardBias;
    LONG m_nDaylightBias;
    SYSTEMTIME m_stcStandardDate;
    SYSTEMTIME m_stcDaylightDate;
} TZI;

Look up MSDN's TIME_ZONE_INFORMATION page (http://msdn.microsoft.com/en-us/library/ms725481(v=VS.85).aspx) for how to interpret the Bias fields, and especially the StandardDate and DaylightDate fields -- they are gently abused to support constructs like "the last Saturday in April".

HTH