Time Conversion

• Get struct timeval
  gettimeofday()
• Get struct timespec
  clock_gettime()
• Time string -> struct tm
  strptime()
• struct tm -> time string
  strftime()
• tm -> unix time time_t
  mktime()
• Others.
  Refer time.h
  (asctime() , clock() , difftime() , gmtime() , localtime() , mktime() , strftime() , strptime() , time() , utime())