With an Annual Multi-Trip policy, because you're covered for 365 days, you get to decide when you wish your cover to begin. The date can be the day you purchase the policy, so you are covered immediately for cancellation, or it can be later on. Bear in mind that you wouldn't be covered for cancellation until that date.
For Single Trip policies, your cancellation cover begins immediately after payment, and all other benefits on the policy come into effect on the day you travel.