There is a website called
oh, don't forget... which I think is kinda what you are looking for. The only downside is that the SMS is sent from 'reminder@ohdontforget.com' instead of your number. You input any mobile number, when you want the text to send, and what you want the text to say. I use it to send myself reminders all the time, and I see no reason why you couldn't use the site to send others messages. The interface is simple - you just type in "tomorrow, 6pm" or "now" or something like that for when you want the text sent. It's a very useful site. Hope this helps.