std::difftime (3) - Linux Manuals
Defined in header <ctime>
double difftime( std::time_t time_end, std::time_t time_beg );
Computes difference between two calendar times as std::time_t objects (time_end - time_beg) in seconds. If time_end refers to time point before time_beg then the result is negative.
time_beg, time_end - times to compare
Difference between two times in seconds.
On POSIX systems, std::time_t is measured in seconds, and difftime is equivalent to arithmetic subtraction, but C and C++ allow fractional units for time_t.
// Run this code
duration a time interval