If I understand correctly, you're partially right. It is a year from whenever purchased, so if you converted your warrant on December 31, 2019 then you would be taxed short terms capital gains until December 31, 2020. If you sold January 1, 2020 then you'd have to wait until January 1, 2021. Overall, selling in 2019 didn't make a big difference if you did it at the end of the year if you think that the company might get bought out anytime prior to 1 year before you exercised the warrant.