You can schedule Emails and SMSs from within conversation either under the Conversations menu link or the Contact View.  

Inside the conversation section, click on the sending a message or email. After typing the message content click on the clock next to 'Send Now'.

You can then select the date, time and timezone for this message to be delivered on.

Once the date and time is setup, click on schedule and then click on Send Later to queue the message.

Your message will be queued to be sent on the date and time it was scheduled for.

To cancel this scheduled message, just hover over the message and click on the details button. At the bottom you can see the option to 'Cancel Send', click on it to cancel this scheduled message.