Before now, users make use of third party sites like tweet deck or Hootsuite to schedule their tweet, but now, Twitter has rolled out the ability to schedule your tweet till when you want it to be sent.
Along with this feature, is an ability to save draft. If not yet ready to post a tweet, you can just save it as draft.
Tweet Scheduling typically allows users to table their tweets to be sent at a specified time after being drafted and is effectively the bread and butter of many social media managers.
According to Twitter, you can draft or schedule it to send at a specific time –– all from the Tweet composer!
— Twitter Support (@TwitterSupport) May 28, 2020
This feature is currently available to web app only and not yet available on mobile devices. Mobile users will still have to make use of third party sites mentioned above.