I'll have to admit I didn't get as deep as differential calculus!
--
===
I studied calculus in engineering school but never considered myself
to be highly proficient at it. A few years later I was on the
technical staff of a high energy physics lab at Cornell University. On
of my colleagues was a PhD EE who could solve differential equations
in his head. That always amazed me.
Certain aspects of electrical and electronics engineering involve the
use of differential calculus equations. So does the vacuum equipment
industry I was involved in.
I can do the calcs (carefully) when I understand what I am solving
for .... like pumping calculations in a system that passes through
viscous flow to non-viscous molecular flow. But to understand
the math involved just for the sake of understanding it
is my weakness.