Hi Yasmijn,
It's lovely to see that you're planning another visit to the Walt Disney World Resort.
You are quite right in saying that you are now required to enter your arrival date when purchasing
Ultimate Tickets from the Walt Disney Travel Company (www.disneyholidays.co.uk) based here in the UK.
According to the terms and conditions on the Walt Disney Travel Company's website, the 14-Day Ultimate Tickets still expire 14 days from first day of use.
2016 tickets are for arrivals between January 1, 2016 and December 31, 2016 and must be used for the first time by December 31, 2016. Likewise, 2017 tickets are for arrivals between January 1, 2017 and December 31, 2017 and must be used for the first time by December 31, 2017.
So long as you enter your actual arrival date when purchasing your 14-Day Ultimate Tickets, I don't see there being any issues.
If, however, you wanted to clarify this with a Cast Member prior to purchasing your tickets, I would recommend calling 00 44 800 16 90 730.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma