My recent posts on the Ablative of Agent sent me back to my earlier work on the Gerund/Gerundive distinction, and I realize that I never fully articulated that gerundives take a Dative of Agent.
- This province is for you to defend: haec vōbis prōvincia est dēfendenda.
- I have to fight one thousand matches: mihi est mille certamina pugnandum.
- The armor you must wear is in the cabinet: arma gestanda tibi in armāriō sunt.
AG would like us to compare this construction to the dative of possession, viewing the gerundive as an ascribed duty for the dative Person:
- I have to fight one thousand matches: mihi est mille certamina pugnandum.
- My name is Commodus: mihi nomen Commodus est.
With the Second Passive Periphrastic construction (always gerundive + sum), the Ablative of Agent (ā/ab + ablative) may appear where a dative would be ambiguous.
- To whom must you submit: quibus est ā vōbis cēdendum?
- To whom should we give the books: quō ā nōbis librōs legendōs sunt?
The Essential AG: 374
My earlier work on the Gerund/Gerundive divide starts here. Flip posts at the bottom of the page to find the one you want. There are five of them on the topic.
Latin for Addicts 
You dropped the preposition from the English translation of the third example: “The armor you must wear is [in] the cabinet.”
Nice blog. I only recently discovered it (by searching for help on the Greek declensions in Latin), but it’s already helping me review and refine my Latin grammar. Thanks!