Implementations of this specification provide for monitoring time certificate generation requests. An example method performed by a database service that stores data in a blockchain includes, in response to receiving a time certificate generation request, determining a starting block height H1 and an ending block height H2 of a target ledger of the blockchain corresponding to the time certificate generation request; in response to determining that the starting block H1 of the target ledger is greater than a block height H of time authenticated data blocks maintained by the database service, executing the time certificate generation request; and in response to receiving a time certificate generated by a time authentication service for the time certificate generation request, changing a value of the block height H of time authenticated data blocks maintained by the database service to the ending block height H2 of the target ledger.