1
general::
Time passes quickly, as in It's midnight already? Time flies when you're having fun, or I guess it's ten years since I last saw you? how time flies. This idiom was first recorded about 1800 but Shakespeare used a similar phrase, "the swiftest hours, as they flew," as did Alexander Pope, "swift fly the years."
American Heritage Idioms
2
general::
Phrase(s): time flies (when you’re having fun)
Fig. time passes very quickly. (From the Latin tempus fugit.) • I didn’t really think it was so late when the party ended. Doesn’t time fly? • Time simply flew while the old friends exchanged news.
McGrawhill's American Idioms And Phrasal Verbs