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dt unit is time (seconds) as you see it defined in the first line.

 dt=(current_time - last_time).toSec();

It might seem odd without some theoretical background but vth describes an angular speed and it's unit is rad/s

vth = (speed_right - speed_left)/lengthBetweenTwoWheels;

It is explained here , simple trigonometry.

And if you double check, vth unit as per that equation is not m/s but just s^-1 (or 1/s) which is actually correct, think about rpm (revolutions per minute) whose unit is 1/s (revolutions don't have a unit)