Nyzo uses a fixed, 7-second block duration. The "open edge" is the latest block that is available for the blockchain to process. When the cycle is functioning optimally, it will freeze blocks so quickly that there will be a period of waiting between the freezing of a block and the time at which the next block becomes the open edge. This means that the frozen edge will be 0 or 1 blocks behind the open edge at all times.
When the cycle stalls or is not producing blocks quickly enough, the frozen edge can fall far behind the open edge. Previously, such a state would cause new transactions to be delayed by approximately the amount of time between the end timestamp of the frozen edge and the end timestamp of the open edge.
The web wallet has been modified to eliminate this delay. When the frozen edge is far behind the open edge, the web server now modifies the timestamp of the previous-block hash response to compensate for the blockchain processing delay. This causes the transaction to be produced with a timestamp just past the current frozen edge. While this may seem to be an odd manipulation, the mechanics of the blockchain do allow this, and there is nothing to be gained by timestamping transactions with the current timestamp instead of a timestamp in the past.
The ability of the cycle to process transactions in a timely manner is critical to the utility of the system. The verifier-removal process introduced in build 508 will help to ensure the health of the cycle and maintain processing throughput, but we would be naive to assume that stalls or slowdowns will not occur in the future. This timestamp change will drastically reduce the impact of stalls and slowdowns on the usefulness of Nyzo as a transactional system.
Also, any verification of transaction timestamps to prevent the submission of past timestamps would require a consensus process essentially equivalent to the block consensus process. So, the only reasonable way to ensure exclusion of past-timestamped transactions is to freeze blocks quickly.
This does raise the question of what transaction timestamps mean in Nyzo. Simply, a transaction timestamp is the timestamp that the sender of the transaction chose to assign to the transaction, nothing more and nothing less. This timestamp determines block membership. If a transaction is incorporated into the blockchain, then we can trust (as much as we trust the blockchain) that the transaction was produced before or close to the verification timestamp of the block. However, there has never been a guarantee that a transaction included in a block was produced before the end timestamp of that block.