I use calculus quite a lot, basically any time I need to go beyond an existing solution I could look up in a textbook. That is, all the interesting problems! For example in writing this article, especially details in footnotes 4 and 13: https://jbconsulting.substack.com/p/voiding-the-warranty-how...

When I want to write or tune a Kalman filter or some other control system, I need to work out the differential equations that govern a system, often from first principles if the system isn't a simple textbook example like an inverted pendulum. Sometimes I need to design that system myself or find out what change to its dynamics will fix a noise problem (eg. add a capacitor here, etc), and without an understanding of calculus I would be searching in the dark or stuck trying to look up known solutions in textbooks.

It also comes up a lot when I want to do probability or statistical calculations, because I end up needing to integrate various functions or products of functions to find means / medians / covariances / etc. Sometimes I can get away with leaning on Mathematica to do those integrals for me, but sometimes I can't, like when one of those functions is an arbitrary input like f(x). Even when I can use Mathematica to perform the integral, I need to know calculus for that to be any clearer than a magic spell. Without knowing calculus, how would I ever guess that I can solve my problem by typing "ln(x0-a)/arctan(x + a)" into wolfram alpha and then plotting the result?

Recently I had the need to do work backwards: find the function that satisfies certain properties like - being smooth and monotonic, zero at the origin, having a certain area within certain bounds, having a certain covariance, nowhere going to infinity, etc. This sounds very abstract but was the key to cracking a concrete signal processing problem.

And then there's all the optimization work, where I need to write numerical code that will look for the maxima or minima of some unknown function, to eg. design the best possible motor you can pack into a box of certain size and weight. 3rd-party libraries are certainly helpful but I usually find I need to supplement them with domain logic to keep it from getting stuck in a local optimum or rule out physically impossible search spaces.

A few months ago I needed to write a finite-element solver to handle soft, thin woven fabrics, where the material can support tension but not compression stresses and the material properties are different depending on the axis of the weave. I would have been at a total loss to get that done without calculus.

Even when I am pulling something out of a textbook, most engineering textbooks past the first-year-undergraduate level will presume some familiarity with calculus.

Not to mention properly thinking about current issues like inflation, CO2 emissions, etc, where the discussion bounces back and forth between integral and derivative quantities.

I'm not even a "math guy". I didn't do well in math in school, and I always struggle a lot to learn new math until I have an application in mind that I need it for. It's only because of how often I need it for my projects that I've ended up learning a lot of math, not the other way around.