noun
leap second
A one-second adjustment occasionally applied to Coordinated Universal Time (UTC) to keep it in sync with astronomical time.
今年の6月30日に閏秒が挿入される予定です。
A leap second is scheduled to be inserted on June 30th this year.
閏年 refers to a leap year, while 閏秒 is a leap second, a much smaller time adjustment.
Compound of 閏 (うるう, 'leap') and 秒 (びょう, 'second'). The term is a direct calque of English 'leap second'.