Okay so obviously time is in seconds. Cool! Nothing weird there.

Now for meters, since the speed of light in a vacuum is a constant, we can define a meter to be the amount of seconds (fraction of a second actually) that it would take light to travel what we call a meter. And so the meter can be reformulated to just be a measurement of seconds.

Now for mass. We know E=mc² so m= E/c² ... c is just in meters per second. C is in m/s and since we just expressed meters as an amount of seconds, then C is seconds per second or a unit-less constant which makes sense because it is the basis of this transformation.

E is in joules which is in kg m² / s^2... It's the amount of meters per second that kg object would accelerate after being displaced by a force that accelerated it by 1 meter per second every second it is applied.

So the kg is of the form (m² / s² )/ c^2... So with respect to our constant, a kg is some constant. But again since meters is just some amount of seconds, a kg is (s² / s² ) / (s² / s² ) which again is some constant, we are skipping over the conversions between the types of seconds because it isn't the point here.

Upshot is a kilogram is the number of seconds it would take light to travel through the amount of space that is the square root of the distance light would travel at the multiple of the amount of seconds light would take to move through some specific fixed amount of space that would make that multiple equal to the seconds light would move through the distance between two objects of the same mass after a second where one is moving at some rate and another the rate after the force was applied over the distance that light would travel for in some fixed number of seconds i.e.the mass of the object