"It is one o'clock."
When pronounced as hitotoki, it's typically written as ひととき or ひと時, and it means "moment" or "a short time". An example is: 「ひととき休憩（きゅうけい）を取（と）りましょう」= "let's take a short break" (lit. Let's take a break for a short while)
When pronounced as ichiji, it means "one o'clock". To use the above example: 「一時に休憩を取りましょう」= "let's take a break at one o'clock"
If I'm being pedantic (which I am f(^_^; ) both are now Japanese readings.
To properly answer your question, ジ is the on'yomi, or the reading derived from Chinese pronunciation and とき is the kun'yomi, or the pre-existing Japanese pronunciation.
I make the distinction because, as far as I know, there is a large number of kanji which are pronounced very differently from the modern Chinese pronunciation, and some kanji which Chinese doesn't even use at all any more.
Yeah, people usually think of kanji as being the same as Chinese characters. I studied Chinese before I started learning Japanese, so I expected kanji to be really easy. I was surprised to learn the kanji for "I/me" isn't 我 like in Chinese, but one I'd never seen before 私.
Which is why I was being a little dick-ish about making the distinction of kanji and on'yomi being derived from Chinese, and not the same. Sorry f(^_^;
分 (fun/pun) means "minute" (in the context of time) and it's only used when you want to specify the minutes.
Just like in English, you don't say "1 o'clock and zero minutes"; it's not necessary. Likewise in Japanese, 一時 means "1 o'clock" and you don't need to say how many minutes.