Themes
Happy the man, and happy he alone, He who can call to-day his own; He who, secure within, can say, To-morrow, do thy worst, for I have liv'd to-day.
— John Dryden · Imitation of Horace

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker says only one who can call today truly his own is happy: someone settled enough to tell tomorrow to do its worst, having already lived fully today. It praises contentment in the present and defiance toward whatever the future may bring.
Context reviewed