When setting up a campaign, you selected the option to "Send all at the date and time specified", then set a date in the future as well as a time. But, the mailing sent now. Why did this happen?
The most common cause of this issue is that the "Send Now" button was clicked instead of clicking the "Save" button. Be sure to click the appropriate button (Save) to save changes to your campaign.
The image below shows the schedule page of the create a campaign workflow and the placement of the buttons on the page.