This change is to replace vote sequence numbers with unique timestamps. https://github.com/nanocurrency/nano-node/pull/2879
Currently, each vote contains a sequence number that is used to identify the most recent vote from each representative. This number is started from 0 and incremented for each vote generated. The sequence number is used to prevent vote replays from being processed as only the highest sequence number for each election is considered when confirming a vote.
Sequence numbers are simple, however, the number needs to be maintained for the lifetime of the representative. The sequence number is periodically saved to disk so a representative can recover it when a node is restarted. In the case that a representative key is moved to a new ledger, or the stored sequence number is otherwise lost, nodes on the network will replay the most recent vote they've seen from this representative back to them, so the sequence number will be synchronized.
Using a timestamp as a vote sequence number allows nodes to use local clock synchronization in order to generate a unique timestamp without the need to record the latest sequence number to disk, and without needing functionality for nodes to replay votes in the case that a representative key has been moved.