Need differential equation of this circuit

All elements in this circuit has zero initial values.
I need to know at what time the voltage level on capacitor C1 passes a certain level.
Vs(t) is DC in my implementation. I want to know at what time C1 charges to 20V (or any other voltage).

Unless this is homework, the easiest way to determine that is with a Spice simulator such as LTSpice.