残業: negated overtime and an unexpected delay
Read 残業せず with leaving on time and 急な残業 with lateness for dinner, preserving negation and avoiding a nighttime-only interpretation of overtime.
残業せず: no overtime in this example
残業 (ざんぎょう) means overtime. 「今日は残業せずに定時で帰った。」 means “Today I left work on time without working overtime.” 残業せず negates working overtime. The sentence also says the speaker left on time, so it must not become a story of doing overtime and leaving late.
せず is a negative form related to する. Together with leaving at the scheduled time, it establishes no overtime on that day, without establishing that the speaker never works overtime.
急な残業: unexpected work and lateness
「急な残業で夕食の約束に遅れた。」 means “Unexpected overtime made me late for my dinner appointment.” The unexpected overtime caused lateness for a dinner appointment. Lateness is the stated result; the sentence does not say that all work was finished or that the appointment was canceled.
急な emphasizes that the overtime arose unexpectedly. This situation is separate from leaving on time in the first example, rather than a contradictory account of a single day's work.
Read only the time information provided
The Korean translation uses a term commonly rendered as night work, while the English uses overtime for these same examples. Read 残業 here as work beyond the regular schedule without making nighttime a condition of every use.
The word alone does not set exact start and end times or an actual working-hours standard. The supplied information is negated overtime, leaving on time, unexpected overtime, and lateness for dinner.
How do 残業せずに定時で帰った and 急な残業で夕食の約束に遅れた differ in overtime and outcome?
Show the answer and why
The first reports leaving on time without overtime; the second reports unexpected overtime causing lateness for dinner. They are separate examples, and neither licenses an added claim about work completion or cancellation. 残業 is not limited to work at night.