Wait that's not even right. The meter is NOT defined in terms of 'some rod'. It used to be in terms of a platinum-iridium bar, but now it is defined as the distance light travels in --very small but specific amount of time--. The speed of light in a vacuum is a constant, defined to be EXACTLY 2.99*10^8 m/s, though the actual number is written out to more digits. Instead of measuring more precisely the speed of light, we measure more precisely how long a meter is.
Wait, how long is a second? I'm glad you asked. It's the amount of time it takes a caesium-133 atom to oscillate a whole bunch of times, or something like that.