flux = 4.5*sin(6.281*t)
dfluxdt = d(flux)/dt
to get the derivatives?

The above is symbolic calculation. EJS or java did not have such function yet. May be there are some java library available.

If you already [b]flux = 4.5*sin(6.281*t)[/b].
This is a mathematic equation.

Then you can [b]dfluxdt=4.5*6.281*cos(6.281*t)[/b] to get the derivate.
It is just calculation. I do not see any modeling activity above.

if you define flux as variable and calculate it's value in your EJS model. You can calculate d(flux)/dt in EJS and compared it with theorical formula with data tool. It is modeling activity.