In my opinion the best way to perform date/time calculation is to rely on SDK functions.
SystemTimeToVariantTime transforms a SYSTEMTIME structure into a variant time, i.e. a decimal number whose integer portion is related to date and whose fractional portion relates to time. VariantTimeToSystemTime performs the opposite operation, returning the value in a SYSTEMTIME whose elements are day/month/...
Once in variant time, you can add, subtract, etc time and date with the corresponding algebric operations. It's basically what excel does with Date/Time format.
To use these functions you will need to include in your project windows.h and oleauto.h, and to add oleaut32.lib.
You find an exhaustive description of time functions inside the SDK help: look in the TOC for Management services >> Windows System Informations >> Time.
