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From TurboActivate.h:
/* Checks if the string in the form "YYYY-MM-DD HH:mm:ss" is a valid date/time. The date must be in UTC time and "24-hour" format. If your date is in some other time format first convert it to UTC time before passing it into this function.
Returns: TA_OK on success. Handle all other return codes as failures.
Possible return codes: TA_OK, TA_FAIL, TA_E_INVALID_FLAGS, TA_E_INVALID_HANDLE*/TURBOACTIVATE_API HRESULT TA_CC TA_IsDateValid(uint32_t handle, STRCTYPE date_time, uint32_t flags);
Notice you're missing the handle.