I emailed Tim but he didn't reply yet? I'm going to take an educated guess and say 3 weeks prior to the event. That would make the cutoff Thursday March 22nd 2018.
Based on previous years info, I'm pretty sure Tim would update his website to this:
This all day track event is $325. Pre-register before 3/22/18 and pay only $275.
No refunds after on time registration date 3/22/18, which is 3 weeks prior to event. Same Day Registration add $50. So that’s $375 same day for you disorganized, uncommiting types. No credits or refunds after 3/22/18."