A dielectric of permittivity $3.5 \times 10^{-11} \mathrm{~F} / \mathrm{m}$ completely fills the volume between two capacitor plates. For $t>0$ the electric flux through the dielectric is $\left(8.0 \times 10^{3} \mathrm{~V} \cdot \mathrm{m} / \mathrm{s}^{3}\right) t^{3} .$ The dielectric is ideal and nonmagnetic; the conduction current in the dielectric is zero. At what time does the displacement current in the dielectric equal $21 \mu \mathrm{A} ?$