I want to use the plan builder to build a plan up until April/may next year when I’m outside more but I don’t have a specific event to aim for so I use an arbitrary end date.
This means the builder adds in the specialist section which I understand is the final sharpening before crushing everything on the day. I don’t need this really so I’ve previously messed around until I have enough build/base then delete the last section but it would be easier if I could just have build/base and end date.
Just do base and build without plan builder then
That’s is what I have been doing but with the push for plan builder and the ability for it to make a plan with suitable number of base and build weeks to fit into the Length of time you have it would be a nice option.
Trick Plan Builder by picking a date a couple of weeks after your desired base/build completion date and just don’t execute the specialized phase. You’ll probably have to play around with it to experiment but should get you what you’re looking for.
I’d put in a full base-build cycles manually, which will take you to mid-february. Hopefully by then you have a better idea how the 2021 season is likely to shape up, and can use the plan builder at that stage.
Yeah, thats my plan. More just a nice idea for making plan builder useful to all riders.
You can definitely do what your fellow athletes are suggesting by repeating Base and Build, but there’s a lot to be said for completing a Specialty phase! While it is meant to be incorporated with events, you’re still doing a lot of fitness gains in Speciality phase, and you’re getting a lower volume of training which will be a nice change of pace and prevent burnout from the higher volume Base and Build! Maybe worth considering since you have plenty of time before April/May. 
Really? Well every day is a school day!
I’ll use the builder than, thanks @IvyAudrain for the advice.