Because there are no units, the equation can only be used to compare one rate of work to another rate of work at a different time (or a different deadline).

ratio of rate of work at time t1 compared to rate of work at time t2

= rate_of_work(t1, d) / rate_of_work(t2, d)

= (1 / (d - t1)) / (1 / (d - t2))

= (d - t2) / (d - t1)

This works because the only variables left are in the same units, even if that unit itself is unspecified.