How/when is calculus used in Computer Science?
I can think of a few courses that would need Calculus, directly. I have used bold face for the usually obligatory disciplines for a Computer Science degree, and italics for the usually optional ones.


Computer Graphics/Image Processing, and here you will also need Analytic Geometry and Linear Algebra, heavily!  If you go down this path, you may also want to study some Differential Geometry (which has multivariate Calculus as a minimum prerequisite). But you'll need Calculus here even for very basic things: try searching for "Fourier Transform" or "Wavelets", for example -- these are two very fundamental tools for people working with images.
Optimization, non-linear mostly, where multivariate Calculus is the fundamental language used to develop everything.  But even linear optimization benefits from Calculus (the derivative of the objective function is absolutely important)
Probability/Statistics. These cannot be seriously studied without multivariate Calculus.
Machine Learning, which makes heavy use of Statistics (and consequently, multivariate Calculus)
Data Science and related subjects, which also use lots of Statistics;
Robotics, where you will need to model physical movements of a robot, so you will need to know partial derivatives and gradients.
Discrete Math and Combinatorics (yes!, you may need Calculus for discrete counting!) -- if you get serious enough about generating functions, you'll need to know how to integrate and derivate certain formulas. And that is useful for Analysis of Algorithms (see the book by Sedgewick and Flajolet, "Analysis of Algorithms"). Similarly, Taylor Series and calculus can be useful in solving certain kinds of recurrence relations, which are used in algorithm analysis.
Analysis of Algorithms, where you use the notion of limit right from the start (see Landau notation, "little $o$" -- it's defined using a limit)


There may be others -- this is just off the top of my head.

And, besides that, one benefits indirectly from a Calculus course by   learning how to reason and explain arguments with technical rigor. This is more valuable than students usually think.

Finally -- you will need Calculus in order to, well, interact with people from other Exact Sciences and Engineering. And it's not uncommon that a Computer Scientist needs to not only talk but also work together with a Physicist or an Engineer.