It is pretty simple to figure out the beginning date of travel insurance. The day that your trip protection goes into effect is called the Effective Date, and this date varies based on the date of your vacation and what kind of plan you bought.
This type of plan has it all—it offers you inclusive coverage, and you can still be covered in cases of trip cancellation and medical evacuation. When you buy a comprehensive plan, the date the plan goes into effect is one minute after midnight the next day after your purchased the plan.
If you did buy your insurance on May 10, at 4:12 pm, then your insurance will go into effect on May 11, at 12:01 am. This means that if you need to cancel your trip for a covered reason, you can, and you can be sure that you get the right coverage for you.
Everything else, including post-departure benefits, is not in effect until you leave for your vacation.
Travel Medical, Accidental Death, or Medical Evacuation: A Different Plan
These plans are different. They do not have medical evacuation coverage, so they do not go into effect until the day you leave for your trip. If you bought something on May 10, and your trip begins on June 11, then your coverage begins on June 11.
More Than One Trip: A Different Plan
The date this plan goes into effect is the date of the first trip planned. Be sure that you check with the company that you booked your insurance plan through for this one. You can always double check with the insurance company, and you can read the plan to make sure that you get the right trip protection or travel insurance.
So, as you can see, the inclusive coverage option is the best one for cancellation and the coverage begins the day after you buy the policy. For non cancellation related coverage, the coverage begins when your trip begins.