CPU Execution Time

I am using mkl_get_cpu_clocks() function to measure the CPU execution time of LAPACKE_dgesv. My code is like:

unsigned MKL_INT64 t1,t2;
mkl_get_cpu_clocks(&t1);
run LAPACKE_dgesv()
mkl_get_cpu_clocks(&t2);
printf("Elapsed MKL: %lf  \n", (double)(t2-t1)/1000000000);

However, the returned elapsed time is not even close to the execution time. The actual execution time is about 27 seconds while the returned time is about 64 seconds!

I checked the Intel MKL manual and unfortunately there is not enough discussion about mkl_get_cpu_clocks. Can you please help? Thank you.

-Afshin