Which word to use for "day" when talking about lengths of time?

Question

I know of the words 日 and 天, which both mean "day" in Chinese, but I'm not sure which to use in phrases such as "I've been in Taiwan for 25 days".

When I look in Wiktionary I'm even more confused:

• 日 is marked "literary or dialectal"
• 天 is marked "Obsolete sense in Modern Standard Chinese - may be preserved in fossilised compounds and expressions or other varieties of Chinese."

So which should I use for saying things like "I think I will stay two or three days"?

Maybe there's a third word I don't know about?

Answer 1

Both of the "天" and "日" as the "量词" are correct.

In the modern standard Chinese, people will say "我觉得我会待两三天" in their daily life, and will say "我预计我将停留两三日" in the formal situation such as applications or official documents.

Answer 2

Basic difference:

天 (n) sky; day

日 (n) sun ; day; date

When stating number of days, you can use either 日 or 天

Example:

三個月另八日 /三個月另八天 ( three months and eight days)

十六日 / 十六天 ( 16 days)

For non-specific day, you can use either 日 or 天

Example:

今日，昨日，明日= 今天，昨天，明天 (today, yesterday, tomorrow)

這一日 = 這一天 (this day)

那一日 = 那一天 (that day)

today, yesterday, tomorrow, this day and that day can be any day on calendar.

When stating a date on calendar, use 日 or 號

Example:

三月八日 / 三月八號 ( 8th March)

you cannot say 三月八天

When stating a specific date, use 日

Example:

國慶日, 開放日 ( Independent day, open day)

you cannot say 家長天, 開放天

In summary:

• When stating number of days, 天 and 日 are interchangeable
• For non-specific date, 天 and 日 are also interchangeable
• When stating specific date, use 日

but I'm not sure which to use in phrases such as "I've been in Taiwan for 25 days".

Since you are referring to the number of days, you can use either.

Answer 3

For daily usage I suggest you use "天", but "日" is still widely used in literacy (especially old books) and on formal occasions.
For example:
扬州十日 (A )Ten Day('s massacre) in Yangzhou
我觉得我会呆两三天。 I think I will stay two or three days.

EDIT: More examples for you:
过年七天乐(a popular TV program) 7 Days' Fun on New Year
北京一日游 a one-day tour in Beijing
And you may always say 天 in daily use instead of 日:
给我十天时间就够了。 Ten days are enough for me.
本航班需要航行一天又六个小时。 This flight will take one day and six hours.
Plus, for Sunday, 星期天 is used in spoken and informal languages, while 星期日 in formal:
星期天一起出去玩好吗？ Shall we go out on Sunday?
截止日期为星期日晚九点。 The deadline is 9 p.m, Sunday.

EDIT: For your example, the correct word is "天".
我已经在台湾呆了25天。I've been in Taiwan for 25 days.

Answer 4

You should use "天" here, since it needs "量词(unit)" here.

Both 日 and 天 could be used as noun like "day" in English. e.g. 星期天／星期日 are both correct for "Sunday". But you should always use 天 when you try to express "a number of days"， like 十天，三天，etc.

I think I will stay two or three days. 我觉得我会待两三天。

But there are exceptions as well. When you want to use a phrase which contains days, usually you should use 日 but not 天。 e.g

我们报了一个新马泰七日团。 We ordered a Singapore-Malaysia-Thailand seven-days group tour.