I googled it and found an identical question on Quora, to which someone gives the following answer:
"My thought is: If you don't know what you're doing right now, don't waste more time spinning your wheels without a clue. Figure out what you want and do it."
I get the general gist of it;...
By "you are not sure of a minute", I think he means that nobody has any guarantee of any time remaining to them. It's a recognition that life is very uncertain. Every minute could literally be our last.
Thank you all for answering.
Don't waste time?
Work when fruitful ---> leisure.
If one has not secured leisure, one ought recognize the power of now and capitalize on it--carpe horam.
There's a touch of Hume's questioning of causation in Ben Franklin's quote; likely Franklin knew of Hume's work; Ben Franklin was born about 5 years before Hume, and enjoyed a long lifespan.
Related: "The Power of Now;"
"Man, Master of His Destiny;"
"The Path of the Higher Self."