Google will be offering a new feature in its Messages app and some users can already take advantage of it. They can now schedule their messages to be sent as shown in the image below.
To activate the function, all you have to do is keep your finger on the send message button. It then offers three hours of sending, or to choose the date and time. Note that sending can only be done if the smartphone is on. This option works with traditional SMS and does not require the RCS (Rich Communication Services) protocol.
A clock icon will then appear next to the message to let you know that it is deferred. You have the option to edit, delete and send it immediately.
The deferred sending function will be rolled out gradually and is added to the new function organization of messages that Google has just added in its application.
Source : XDA Developers