calculus and analysis – Multiple derivative of unknown function from time-dependent parameter

I am working with the unknown function $j$, which depends on the parameter $phi(t)$, i.e.

$J=f_1(phi(t))+f_2(phi(t))$.

I find derivative $frac{d}{dt}(frac{dJ}{dtheta(t)})=?$ in implicit form and get formula:

J = f1((Theta), t) + f2((Theta), t)
D(D(J, (Theta)), t)

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Reason: General derivative of $J$ function can be much more complex, and the implicit representation allows it to be represented as a sum of simpler components. These are the components I’m going to explore.

Problem: How do I turn this formula into a programming-friendly one in Mathematica?