意思是一样的。不过不缩写的话比较正式。比如如果写正式的文本，那就尽量不要缩写。建议网上查一下还有那些缩写形式。

Just remember these two rules: (1) it's always means "it is". (2) its is a possessive pronoun。 It is the neuter pronoun that is like the masculine "his" and the feminine "her". In other words his=他的， her=她的， its=它的。 Having said that, I will add that a lot of English speakers make mistakes. In English, using an apostrophe (that's what the ' is called) usually means a possessive. For example, we say "that man's son" to translate 那个男人的儿子 (note the ' in "man's" to indicate possession that gets translated as 的 in Chinese). Since we usually use the apostrophe to indicate possession, people make the mistake of thinking that "it's" is also a possessive. That is wrong. Although you'll probably see a lot of examples written by native English speakers that make this mistake, it is still wrong. Just remember the two rules at the start of this answer. So, to answer your question: yes, you can replace "it's" in the sentence about the clock with "it is" and there is no difference in meaning.

Its always means A possessive pronoun the thing belonging to it. It's always means It is. . As other grammar rules say either or both of these could have a ' you have to have a rule for which one gets it. Above is the rule for English. No ' for pronouns her, his, yours mine theirs, its