How solana calculate Clock::get()?.unix_timestamp;

gistfile1.txt

> It's strongly desired by some users to have a network-controlled best effort timestamp associated with the production time of each block. One such example is https://en.bitcoin.it/wiki/Block_timestamp
> 
> Some important properties of this timestamp are:
> 
> 1. Drift of no worse than a couple hours from the actual block production time
> 2. Replaying the ledger at a later time results in the same block production time
> 3. The block production time is not controlled by a single centralized oracle, but ideally is a function that uses inputs from all validators
> 
> A rough proposal would be to require validators to submit their current UTC time every N minutes. A median time is then constructed from the latest submission and block times are then computed as `last_median_time + slot_offset * 400ms` until the next median time is constructed.
> 
> This issue can be broken up into:
> 
> 1. The definition/implementation of the `getBlockTime` RPC API
> 2. Initial stub implementation where `last_median_time` is fixed to 0.
> 3. Adding time submission to all validators and producing a real `last_median_time`
> 
> 1 and 2 are important to implement quickly to unblock some users

ref
https://github.com/solana-labs/solana/issues/7115