When you go to get your quote online you'll be asked when you'd like your cover to start. This cannot be a date in the past, but you can select either 'Right Away' or 'Later'.
If you choose 'Right away' then your policy will be live as soon as payment is confirmed. Your Insurance Certificate will have the exact date & time that your policy is active on it and is available to download as soon as you've purchased cover, and is also emailed to you instantly.
If you select 'later' you'll be able to select the exact date and time you'd like your cover to start up to 30 days in advance.