1. get current time in the form of struct tm (two function calls to do that)
2. Get number of days from user
3. Add days from #2 above to tm_mday obtains in #1 above
4. call mktime() to normalize all struct tm members.
Note: you can't do silly things like advance number of days beyone the limits of time_t or subtract them to that the result is before 1 Jan 1970. If you want to do that then you will have to use something else, such as write your own function to handle such times.