I'm honestly shocked that with all the ways iphone users are supposed to live in a better world than me, they lack this simple, obvious useful ability that the lowliest SMS/MMS user has.
In America iMessage has extremely strong network effects. I read about an Android user who carried around an iPhone specifically to avoid having a green bubble with dating prospects.
I have honestly never felt the need for this and had to click on a menu that I am ashamed to say I never thought of clicking on but yes, there it is. Scheduled send for SMS. You can even edit the time/ content right there in the UI.