I use TweetCaster onmy android when I am away from my computer to manage the TeamZibbet account. I also use it for another twitter account of mine and have for a year or two now. Here are the steps to schedule a tweet to happen at a designated time in the future.
To write a tweet, you lick on the button that looks like a paper with a pen. This is the same if you are scheduling a tweet for later or sending it in that moment.
Type in what you want yours to say. Pardon the typo, it's past my bedtime and I didn't want to bother retaking... anyways, you type in your tweet and once it is written you touch the top right where there are three dots.
This will open up a menu with the option to Post Later. Dooooo ittttttttt!!
That will open up a calendar to use to schedule your tweet. Just choose the date and time (I of course selected the date and time of the twitter bomb!) and hit OK! Done. After this it will still have your tweet in where you entered it. I just hit back until I get back to my feed.
Pretty simple! Hope it helped.