On the Schedule step (When would you like to send this campaign?):

• Send Now – Immediately send your campaign. The campaign is queued with the current time and goes out on the next run of the background sender.
• Send later – Set up a schedule to send your campaign at a specific time. A date-and-time picker appears (pre-filled with the current time); pick the moment you want.

Then click Send Campaign. Scheduled campaigns are listed under the Scheduled tab with their Schedule Date and can still be edited or cancelled until sending starts. Times use your account's date/time format and time zone settings.