成天 chéngtiān all day long
(colloquial) all day long; all the time
How to remember 成天: 成 to become + 天 day = all day long.
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How it is used
- Colloquial 'all day long / the whole time' - usually with a note of complaint: 他成天玩手机 (tā chéngtiān wán shǒujī) he's on his phone all day.
- Spoken register, interchangeable with 整天 (zhěngtiān) but chattier; often flags an annoying habit.
- Not a duration you measure, but 'constantly, day in day out.'