You can schedule a tweet with TweetCaster iOS using a 3rd party service called Buffer, which has support built into the app.
Type the content of the tweet and then tap "Close" and then select the "Add to Buffer" option. You will then need to sign in with your Twitter or Facebook account or a buffer account. You can then schedule when you want to send the tweet.
We are working to have a native 'post it later' feature in future updates.