If you're using C/C++ then that's not the correct code at all. All TurboActivate / TurboFloat functions return an HRESULT. See TurboActivate.h for the particular success / failure codes.
Are you using C or C++? if so, then this is the correct code:
HRESULT hr = IsDateValid(featureValue, TA_HAS_NOT_EXPIRED);bool stillInTrial = hr == TA_OK;