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