Fixed expression used to remark on how quickly time has passed, often at the beginning of a sentence when reflecting on a past event or milestone. The literal meaning is 'it's a fast thing', but it functions as a set phrase.
Time flies—it's already been a year since I came to Japan.
Time flies—there's only one month left in this year, isn't there?
Standard kanji spelling using 早い (fast/early).
Variant using 物 (thing); less common but seen in some writing.
A fixed phrase composed of 早い (fast/early), もの (thing), and the conjunctive particle で. The literal sense 'it being a fast thing' developed into an idiomatic expression for the swift passage of time.