It depends on the context.
If it is a written statement, 是 is necessary; if it is a verbal statement, especially when the information in concern is known to be the time, such as replying to a question, it is not necessary.
If you do the practice module, it gives you the Chinese sentence and asks you to write it in English. In the Chinese sentence, it has a 是。 If you use the 是 here, it flags you as incorrect. Think it's just an oversight. I reported, also.