first year of the sixty-year cycle (where each year is numbered with one of the 10 heavenly stems 天干[tian1 gan1] and one of the 12 earthly branches 地支[di4 zhi1])
the sixty-year cycle
Definitions (粵典–words.hk)
名詞
六十年；源於古時以十天干同十二地支循環，以甲子至癸亥呢六十種組合為一周（量詞：個） 60 years
Definitions (Wiktionary)
noun
A full or completed set of the sexagenary cycle.
noun
The first term of the Chinese sexagenary cycle, sometimes glossed as "Wood Rat" from its associated terms in the Chinese elements and zodiac.