The latest beta now implements a number of fixes relating to timezone/DST.
It also includes support for using Windows API time functions as opposed to CRT time functions.
The results for the APIs may vary across Windows versions as mIRC uses GetDynamicTimeZoneInformation() from Vista onwards, and GetTimeZoneInformation() for older versions of Windows. There are other APIs for improved DST/timezone support, including historical changes, but it all gets very convoluted.
The API support can be enabled using /timeapi on|off to allow you to compare results between API and CRT calls. This setting affects all features that use times/dates, not just identifiers.
The API versions also support negative pre-unix epoch time values from year 1601 onwards.
This beta is also using thread-safe versions of the CRT functions.
The API support needs heavy testing. If it looks like it is working correctly under different versions of Windows/WINE/etc., it will be permanently enabled. On the other hand, if it looks like it is causing more issues than it resolves, the changes will be reverted.