Twitter appears to be testing a new feature with some users.
For some, the PWA has added a new menu to the tweet status area, which contains only one item, the ability to schedule tweets.
Noticed by HTNovo, when clicked the button brings up a screen which allows you to set the time and even region when your tweet should be posted.
You can also see and manage your scheduled tweets from the same menu.
The feature is available to some on Twitter on the Web, the PWA and mobile App.
The feature does not appear to be available everywhere yet, but will likely roll out more widely soon.