Answer:
I can be found in seconds, minutes and centuries, but not in days, years or decades.
If you are unsure how to use one then just ask a flight attendant.
Defends your noggin from the hard pavement
What is it that given one, you'll have either two or none?
Often found in doctor's offices and pirate flags
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.