今年 and 来年: this year and next year
Distinguish calendar-year expressions from lengths of time.
Events within this year
今年 (ことし) means the current calendar year. 「今年、新しい仕事を始めました」 says the speaker started a new job this year.
「今年の冬に引っ越します」 plans a move for this winter. 今年 is not limited to completed events.
Plans for the next year
来年 (らいねん) is the year after the current one. 「来年、留学する予定です」 expresses a plan to study abroad next year.
「来年の春まで待ちます」 says the speaker will wait until next spring. Do not change this into a fixed one-year duration.
Calendar years versus durations
「来年」 and 「一年後」 use different reference frames. 一年後 is one year after a reference point, while 来年 can identify the next calendar year as a whole.
「今年は行けませんが、来年なら行けます」 contrasts inability to go this year with ability to go next year. It does not report a completed visit next year.
When said in December, must 来年 mean twelve months later?
Show the answer and why
No. It refers to the next calendar year. 一年後 expresses a point one year later.