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# Bank Timestamp Correction

Each Bank has a timestamp that is stashed in the Clock sysvar and used to assess time-based stake account lockups. However, since genesis, this value has been based on a theoretical slots-per-second instead of reality, so it's quite inaccurate. This poses a problem for lockups, since the accounts will not register as lockup-free on (or anytime near) the date the lockup is set to expire.

Block times are already being estimated to cache in Blockstore and long-term storage using a validator timestamp oracle; this data provides an opportunity to align the bank timestamp more closely with real-world time.

The general outline of the proposed implementation is as follows:

• Correct each Bank timestamp using the validator-provided timestamp.
• Update the validator-provided timestamp calculation to use a stake-weighted median, rather than a stake-weighted mean.
• Bound the timestamp correction so that it cannot deviate too far from the expected theoretical estimate

## Timestamp Correction​

On every new Bank, the runtime calculates a realistic timestamp estimate using validator timestamp-oracle data. The Bank timestamp is corrected to this value if it is greater than or equal to the previous Bank's timestamp. That is, time should not ever go backward, so that locked up accounts may be released by the correction, but once released, accounts can never be relocked by a time correction.

### Calculating Stake-Weighted Median Timestamp​

In order to calculate the estimated timestamp for a particular Bank, the runtime first needs to get the most recent vote timestamps from the active validator set. The Bank::vote_accounts() method provides the vote accounts state, and these can be filtered to all accounts whose most recent timestamp was provided within the last epoch.

From each vote timestamp, an estimate for the current Bank is calculated using the epoch's target ns_per_slot for any delta between the Bank slot and the timestamp slot. Each timestamp estimate is associated with the stake delegated to that vote account, and all the timestamps are collected to create a stake-weighted timestamp distribution.

From this set, the stake-weighted median timestamp -- that is, the timestamp at which 50% of the stake estimates a greater-or-equal timestamp and 50% of the stake estimates a lesser-or-equal timestamp -- is selected as the potential corrected timestamp.

This stake-weighted median timestamp is preferred over the stake-weighted mean because the multiplication of stake by proposed timestamp in the mean calculation allows a node with very small stake to still have a large effect on the resulting timestamp by proposing a timestamp that is very large or very small. For example, using the previous calculate_stake_weighted_timestamp() method, a node with 0.00003% of the stake proposing a timestamp of i64::MAX can shift the timestamp forward 97k years!

### Bounding Timestamps​

In addition to preventing time moving backward, we can prevent malicious activity by bounding the corrected timestamp to an acceptable level of deviation from the theoretical expected time.

This proposal suggests that each timestamp be allowed to deviate up to 25% from the expected time since the start of the epoch.

In order to calculate the timestamp deviation, each Bank needs to log the epoch_start_timestamp in the Clock sysvar. This value is set to the Clock::unix_timestamp on the first slot of each epoch.

Then, the runtime compares the expected elapsed time since the start of the epoch with the proposed elapsed time based on the corrected timestamp. If the corrected elapsed time is within +/- 25% of expected, the corrected timestamp is accepted. Otherwise, it is bounded to the acceptable deviation.