The Chinese word for day is
白天

báitiān

Translations for day and their definitions

天
1. sky; heavens; heavenly; celestial
天空 - sky -
天黑了。 - It's getting dark. (lit. The sky is becoming black)
2. (mythology or religion) heaven as the abode of the gods or the blessed departed; heavenly
上天 - to go up to the sky; to go to heaven -
天意 - Heaven's will -
3. (mythology or religion) Heaven as an impersonal deity; (often translated as God)
天主 - God -
老天 - god -
我的天啊！ - Oh my God! -
天助自助者。 - God helps those who help themselves.
4. top; overhead
天橋 - footbridge -
天窗 - sunroof -
5. weather; climate
天很冷。 - It's very cold.
如果天晴，我們就去散步。 - If the weather clears, we'll go for a walk.
6. day (24 hours)
今天 - today -
明天 - tomorrow -
三天 - three days -
7. day (as opposed to night)
白天 - daytime -
三天兩夜 - three days and two nights -
8. period of time in a day
五更天 - just before dawn -
天不早了。 - It's getting late.
9. season
春天、夏天、秋天、冬天 - spring, summer, autumn, winter
黃梅天 - rainy season -
10. nature
天災 - natural disaster -
11. natural; innate
天生 - innate -
天性 - nature, personality -
天才 - genius -

日
1. (star) Sun
2. daytime; time between sunrise and sunset
日班 (rìbān) - day shift
Antonyms: 夜
3. day; twenty-four hours
4. everyday; daily; day-to-day
5. day of the month
三.月四日 (sān yuè sì rì) - March 4
Synonyms: 號
6. some day; some other time
7. former times; previous times; the past
8. time; period; age
9. Short for 日本, gloss=Japan; Japanese; Nippo-:
中日詞典 (zhōngrì cídiǎn) - Chinese-Japanese dictionary
10. (obsolete) Short for 日斯巴尼亞, gloss=Spain
11. (Chinese linguistics) the Middle Chinese initial of 日, id=1
12. (dialectal Mandarin, dialectal Jin, dialectal Wu, dialectal Xiang, vulgar, often in abuses) to fuck; to have sexual intercourse with
狗日 (gǒurì) - fucking
13. Only used in (l-self-lite, zh, sc=Hant, 日)磾, a Han dynasty-era personal name borne by Jin Midi (lw, zh, 金日磾) and Ma Midi (lw, zh, 馬日磾, tr=-).
14. Variant of 時

一天
1. n. one day (twenty-four hours)
2. n. the entire daytime of one day
幹gàn 了一天 - worked for a whole day
3. n. someday; one day
總有一天 - there will be a day (when ...)
4. n. (colloquial) the whole day

白天
1. n. daytime; day
白天開大燈 - to turn on the headlights in broad daylight
白天是教授，晚上shang 是禽獸 - professor by day, beast by night