＠Twitter started offering reserved tweet posting function in web browser version on May 29th. The feature was tested in November 2019 and will be officially implemented.
This function is the one that introduced "Schedule Tweet" that was installed in TweetDeck, which is a high-performance client, to the original Twitter. To use it, you can enter the content of the tweet from the usual posting screen and specify the date and time from the schedule button that appears next to the emoji button. You can also check the list of reserved tweets.
Note that currently only desktop and mobile browsers are available. It's unclear if similar features will be implemented in mobile smartphone apps.