They know this because radio metric dating is extremely accurate. There isn't the space or the time to go over it in detail here, but you might as well ask how do they know how far away the stars are. Suffice it to say that an understanding of the sciences of chemistry and physics have given us the tools to figure these things out very accurately.
(and no I don't care to debate it right now, and yes I am a devout Christian)
It is accurate for the material they are testing if it has not been confused with other material. Something there is 1.1.7 years old. Maybe the fossil and maybe not.
Please do not try to confuse the people with scientific facts.
Someone, else within ten posts is going to claim that radio metric dating is inaccurate because they can not narrow the time of death down to the exact date or time of day.