Gather ye rose-buds while ye may.
Old time is still a-flying:
And this same flower that smiles to-day
To-morrow will be dying.
The glorious lamp of heaven, the sun,
This is to emphasize that time continues to move on. “Old time is still a-flying” means nothing can stop the reader or young women from getting older, the implication being that they will quickly lose their beauty.  The construction ‘a-flying’ is archaic, a feminine rhyme that softens the spareness of the language.