How do I get only the messages of the date I want from the channel?

of course.

The api call I used is below.
https://servername.com/api/v4/channels/channel_Id/posts?page=0&per_page=100&since=Unix_Timestamp

Messages that existed after the unix timestamp were expected to be returned. However, much earlier data (ex: timestamp was May 10, 2020, but all data was imported on May 1) was returned.

And I set per page to 100, but I got a thousand data returned.