Current iOS versions can schedule an iMessage to be sent later from the Messages app. The feature is intended for iMessage conversations rather than ordinary carrier SMS in every situation.
- Open Messages and select an iMessage conversation.
- Tap the add button beside the message field.
- Choose Send Later.
- Select the date and time.
- Type the message and tap Send.
The scheduled message remains visible in the conversation before it is sent, allowing you to edit or cancel it. If Send Later is missing, check that the iPhone is running a current iOS release and that the conversation supports the feature.