I've long given up on meditation timers. I go to a room without any digital technology except for a Casio F91W. I have gotten good at gauging how long I've been concentrating for and if I have aversion to continued sitting I just peek at the watch to know how long it's been.
This is a solved problem that costs $0 and I don't have to worry about receiving personalised ads because I'm into 'mindfulness'.