If you are looking to purchase a Single Trip or Backpacker policy, your pre-travel cover will begin immediately after purchase. From the moment your payment goes through, you are covered under the cancellation section of your chosen policy. The dates you have entered are purely indicative of the days you intend to be on holiday.
With an Annual Multi-Trip policy, your cancellation cover begins on the day you choose as the ‘start date’. For example, if you intend to travel on the 10th, and set your start date as the 10th, you would not have pre-travel cancellation cover for that holiday. If you were to set your start date on the 1st, however, you would receive nine days of cancellation cover prior to travel.