Remember the days when we used to source things in nature as a way to track time? Of course not. Or maybe you were a scout when you were still in grade school.
Today even the little ones want access to technology. But the fact still remains that days begin with the rising sun and end with it setting. The bright moon and stars light the sky in remote areas today.
Now imagine primitive man in the tropics. When the moon is full and big, it rises around sunset. Human curiosity and pursuit for logic must have led to counting days between the occurrences of these simultaneous events. That interval is still referred to as a lunar month.
Using this month, primitive man must have counted the lunar month interval between other annual natural events like spring or winter to get a lunar year.