Glyph origins of the character 岁/歲?
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「歲」 (Baxter-Sagart OC:  /*s-qʷʰat-s/, motion of Jupiter > year > years old) was originally comprised of semantic 「步」 (steps, referring to motion) and phonetic 「戉」 (/*[ɢ]ʷat/). Jupiter appeared brightest to ancient astronomers approximately once a year in the night sky.


  「步」 is comprised of two mirrored feet 「止」, indicating the meaning steps, walking.


As a component of 「歲」, 「戉」 was sometimes variously corrupted into 「戈」, 「戊」, or 「戌」. The modern form is from a corruption into 「戌」.


  All four of 「戉」, 「戈」, 「戊」, and 「戌」 depict different types of bladed weapons, so they're quite easily confused with each other graphically in older writings.