“His time”: The time of his misfortune, especially death.

This is saying that mankind has no idea how long he will live. We might start to town to work, and be killed before we get there. Some die very young. It is sometimes an accident, and other times a disease that strikes when we least expect it.

One of the worst losses of a loved one to deal with is when they die suddenly.

We should not say, tomorrow, I will do something. We may not be alive tomorrow. We should say "if God be willing, I will do that tomorrow"