All you have to do, to extend your trial period, is go to Date & Time preferences and set date before you trial period has started. Then open your Sketch.app and set your time back to atomatic.
But this can be really annoying do it over and over. And you can do it from your terminal too! All you need is date command. To set specific date you it with date parameter in following format +%m%d%H%M%y (MonthDayHourMinuteYear).
sudo date 0102030405
So command above will set your Date & Time to Sun Jan 2 03:04:00 +07 2005. Now when you open Sketch.app everything should appear as trial has not expired yet. But you don’t want to have your computer with wrong date & time. So to set it back you can use following command, which will set your time according to apple time server:
sudo ntpdate -u time.apple.com
not work in 55.2 ver