CFA Level 3 (2019): Derivatives - Box Spread

This is from Reading 33 on Risk Management Applications of Option Strategies.

The box spread is a strategy that should earn a risk-free return. It can also be used to assess for arbitrage opportunities.

Errata:
At time 4:40, the calculation should be $13/1.06^(4/12) = $12.75 which is greater than the initial premium $12.60, so the box spread is still underpriced.

[justinhorsman]

Fabian, I believe your math is incorrect for discounting the $13. You are using 3/12 in your calculation but the written formula shows 4/12 (which I believe is correct). Am I missing something?