(Fact of life) How to calculate how many years you'll use on earth

I was also so puzzled and like ‘is this for real' when I heard it, so many people live life as they like because they believe they've known when they'll leave earth and they'll born again when their end time has approach...i have also carried out my research on it but before I post the outcome of my research and how they do calculate it which makes them feel they've know it first I wanna say they've forgotten that the Almighty God is unpredictable so in other words I think they're deceiving theirselves ....watchout for the next post where i'll write my observations and conclusion of my research and how they do calculate it.