ToDos

Buy 100uF X5R 4V capacitors. There are larger capacitances around, but it's not necessarily good to push things to the edge - larger capacitors in the same package means thinner dielectrics which may mean higher leakage. Try several manufacturers, maybe also different dielectrics.
Measure their leakage (and dielectric absorption) - after hours of charging.
Buy the 60nA RTC module.
Connect it with the L4-DISCO, write software to control it, connect the capacitors and charging arrangement.
Test, powerred from the onboard 3V regulator.
If success, use the solar cell as primary source, and build a simple step-up controlled from the 'L476.
If time and money permits, experiment with low-input-voltage stepup circuits for the primary power.

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