measure calculation time for multiplication of two matrices
 

I want to multiply two big matrices using mmult function.
The size of both matrices are 256*256. Each one is stored in a single sheet.
REsult will be stored in the third sheet.
I am wondering how much time it will take.
In the mean time I would to measure the time necessary for this
multiplication.
What is the best way to measure the calculation time? Is there any command
in Excel which can measure calculation time?