Schedule a Slack message to send later

Open Slack and click the compose button at the top of your sidebar

Select the person/channel you need to send your message to

Write your message in the text box

Once you've finished writing your message, click the down arrow or the right of the send button

Select 'Custom time' from the menu that pops up

Input the date and time at which you would like your message to send

Select 'Schedule Message'