ok, can someone confirm my maths for me?

We are heading to WDW from March 4th - March 20th. From what I understand, we would have 18 days from the date I select when purchasing the tickets to use them. Now if I select March 2nd as the start date, the price is almost $400 cheaper than starting them on the 4th. So If I start them on March 2nd, am I right that we would be able to use the 14 park days up to and including March 19th? We fly home on the 20th, so we wouldn't go to a park anyway that day.
thanks!