ledger's close time rather than the actual ledger's close time, which is not known until much later.
So given that we need the parent's close time, why do we put it in the ledger header? There really isn't a particularly good reason. In practice, you need to have the preceding ledger's header in memory to produce the next ledger anyway. However, not putting the previous ledger's close time in the header would mean that if you only had a single ledger, you wouldn't know what the effective time of the transactions was unless you looked at the previous ledger. That would make it harder to understand what rules were applied during order executions or other operations that require a time.
In summary, the decision to use the previous ledger's closeThe surgical tool kit that was used to perform the autopsy on Napoleon’s body at Saint Helena is on display at the Museum of History of Medicine in Paris.
But few people know that those scalpels probably also emasculated the Emperor.

In his last few months on Saint Helena, Napoleon suffered from excruciating stomach pains. Sir Hudson Lowe, the governor of the island under whose control Bonaparte had been confined, dismissed the whole thing as a slight anemia. Yet on May 5, 1821 Bonaparte died.
The autopsy conducted the following day by Napoleon’s personal physician, Francesco Carlo Antommarchi, revealed that he had been killed by a stomach tumor, aggravated by large ulcers (although the actual causes of death have been debated).
But during the autoptic examination Antommarchi apparently took some liberties.

Francesco Carlo