0001Z On the 1st to 0001z on the 17th would only be 384 hrs or 16 days worth of time. They definitely can pattern you to be out to 2359Z on the 17th day no matter when you start on your 1st day.

They aren't saying you'll be out for more than 408 hrs or 17 days worth of time. They're saying if you go by local time at any of our bases you could be patterned to work on 18 different calendar days. Yes it's only 17 days worth of time but it spans 18 local time calendar days. They're just saying that at many other airlines that schedule you in local time out of your base some of our 17 day patterns would be considered 18 day patterns.

Personally I don't think it makes any difference. Schedule in local or Z, we do 24 hr ops so your potential start and end times would just slide a bit but the total time they could use you is the same.