Let $z=a e^t$ then $dz=ae^tdt$, How to define $d \overline{z}=?$

Question

Let $z$ and $t$ be two complex variables such that $z=a e^t$ where $a$ is a (real or complex) constant. Thus, the differential form $dz$ is nothing but $ae^tdt$, i.e., $$dz=ae^tdt.$$ The question is:

How to define the differential form $d \overline{z}$? $$d \overline{z}=?$$