The genesis of the Hedera ledger was when it launched on August 24, 2018. At that time, all of the hbars were created, the nodes started running, and it was able to start processing transactions from authorized people. Since then, the mainnet has been running continuously, and a growing number of people have used it, including those in the two test programs, and a growing number of dapp developers.
The Hedera ledger works by having all the nodes store the current state, such as account balances. When a transaction modifies that state, such as for a cryptocurrency transfer, then all the nodes process the transaction by updating the balances, and can then discard the transaction. Each state update is signed by a supermajority of the nodes (more than two thirds of the stake). So the state evolves securely over time, without having to store a block chain of its history. This state has been evolving securely for the past year, and will continue doing so in the future, without interruption or breaks. So there is always a trusted current state, but no history on the mainnet.
Mirror nodes are free to keep a history, if desired. The mirror nodes have been running off and on over the last year, and have kept a partial history. So their record of history has started and ended several times. In September, various people and organizations, will start running mirror nodes, with the intention of keeping a history that is continuous from that point on. After that point, each Hedera node will refrain from deleting any old transactions or records until they have been transferred to the mirror nodes. This will ensure that mirror nodes can keep a continuous history, without gaps, if they desire to do so.