get Data Time
Retrieves the starting point for message retrieval. This corresponds to the `sentTime` in the message. If no messages exist in the local database, pass `0` for the first retrieval. Otherwise, pass the `sentTime` of the earliest message to fetch the latest `count` messages.
Return
The starting point for message retrieval. This corresponds to the `sentTime` in the message. If no messages exist in the local database, pass `0` for the first retrieval. Otherwise, pass the `sentTime` of the earliest message to fetch the latest `count` messages.