In order to purchase a single trip travel insurance policy, the departure and return dates must be declared. These are the first and last day of coverage, except for trip cancellation which begins the day after a policy is purchased.
If the travel dates change before the trip begins, the policy can be modified. In many cases, there is no difference in premium cost. However, if the premium does increase, the difference can be paid when the policy is modified.
Remember, every policy has a money back guarantee. During this time, a travel insurance policy can be cancelled for any reason.