Are there any studies comparing the click-through rate for various free trial button labels? For example "Start free 30-day trial" vs. "Try now".
It depends on what the free trial is for, who the target is, but it's not a bad idea to use the word "now" in a CTA. The first CTA "Start..." is a bit long, but long isn't universally bad. It does have the word "free" going for it. I always like the use of a personal pronoun. "Start my free 30-day trial" is even better. To improve upon that you could try the word Get instead of start. Get is pretty much a universally effective CTA verb. People love getting stuff.
One concern with free trials is whether people will have to give their credit card info. I don't have the research, but I think there's a higher click through rate for ones that don't require CC. If a CC isn't required, make sure that's somewhere clear on the landing page near the CTA (but not below, as there shouldn't be any relevant copy below the CTA for ADA best practices). Make the no-CC info prominent--it's important.
(Sorry just realized this is from Nov 2017--better late than never on the reply?)