How do DAOs know the exact start/end time of on-chain proposals when it goes by block number vs timestamp? Is it just an estimate like (# of blocks * 12s block time) or am I missing something?

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Not sure about the specific case, but a smart contract can decide based on the time a tx is included in a block if a call is valid or not.

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I meant more-so on the frontend side, like the "Ended" card here https://nouns.wtf/vote/217 

I'm pretty sure the on-chain vote actually ended ~1 hour after the stated time