What it means
Means time seems to pass ridiculously fast, especially when you're busy, distracted, or actually enjoying yourself. You look up from a game, a shift, or a deep chat and suddenly it's dark outside. People often tag it with the full line time flies when you're having fun, but it works on its own for any where did the day go moment.
Usage examples
"Wow, it's already five and we've done nothing but chat and snack. Time flies. Grab your stuff, we've got to go or we'll miss the train."
"Is it midnight already? Time flies when there's deadlines."
"The kids are at uni now, honestly time flies."
"We only sat down for one drink and now the pub's doing last orders. Time flies."
"I swear I started that episode after dinner and now it's somehow 1 a.m. Time flies."
Where it comes from
It goes back to the much older saying time flies when you're having fun, which is attested in English by the late 19th century. The wider image is older still and echoes the Latin tempus fugit. On its own, time flies kept the same idea and became a plain everyday way to say the hours vanished on you.
