延期 and 延長: postponing a time and extending a duration
Compare moving a meeting to next week with adding time for questions, distinguishing a later start from a longer meeting.
Moving a planned time later
延期, read えんき, means postponing a planned time. 会議を来週に延期しました。 says that the meeting was postponed until next week.
開始を三十分延期しました。 says that the start was postponed by thirty minutes. It gives no duration for the meeting itself.
Increasing a duration or length
延長, read えんちょう, means extending time or length. 質問の時間を十分間延長しました。 says that the question period was extended by ten minutes.
展示期間を一週間延長しました。 says that the exhibition period was extended by a week. The period is the object being extended.
What was moved or increased
開始は延期しましたが、会議の長さは変えません。 says that the start was postponed, but the meeting’s duration will not be changed. Starting time and duration are separate pieces of information.
Extending a period may also move its end date later. Still, identify the stated object, such as 開始, 期限, or 期間, to determine what changed.
Does 開始を三十分延期しました。 mean that the meeting will last thirty minutes longer?
Show the answer and why
No. It says that the start was postponed by thirty minutes. It supplies no information about adding thirty minutes to the meeting’s duration.