"Since death is certain and time of death is uncertain, what is the most important thing?"
Just take a moment to contemplate that. Most of us like to shy away from our mortality and pretend that we're never going to die, and the result is that most of us never really begin to LIVE...at least not fully. We just tend to pass the time in a hazy dream-like state, going from one distraction to the next. No real urgency or purpose. Contemplating your death, far from being morbid, is actually tremendously invigorating and liberating. You realise that time is not finite; and, with that in mind, you can really focus on determining what is most important to you in life.
Make it a habit to stop yourself every so often and remind yourself that your stay on this planet is but a short one and then focus on the big question: what is the most important thing for you, right now? This powerful question has an uncanny way of burning through all the layers of superficiality and things you might have thought were massively important but which really aren't (the petty annoyances, grudges and the various mental-emotional states that, though transient, have a way of completely sucking you in and making you identify with them completely).
So just pause right now....remind yourself that you ain't gonna be here forever, and bearing that in mind, ask yourself what is REALLY important.........