If it's in the top section (change displayed value using TRIP), try holding down TRIP instead of INFO?
Hmm. Worked for me (2016 Roadster) on the bottom line. I don’t normally have that info displayed but I turned them both on, on the bottom line, and long-pressed INFO until the timer reset and starting counting up again from zero, for each of those two timers. I wonder if you have a wonky button? I don’t think the 2015 model is any different in how that works than my 2016.It's on the bottom and I've been trying that procedure with no success. I'm going to trying moving the info to the top display and see if it works any better there.