almost! If you want to say "time","weather","temperature" , you can omit "それは". for example It's nine o'clock:9時です。 It's sunny today. :今日は晴れです。 so, you can say "午後5時です".

When you say it is - time , then you probably had better to add 今（いま）　which means now, but in case you are asked what time it is now,or what time whatever the issue you are talking about then you can say 午後五時です。

it's 5:00PM = 午後5時00分です（Japanese notation） = ごご ごじ です（If I say） it = それは ⇒In this case the Japanese does not pronounce it.

こんにちは　マリオさん。 As menow03さん said, "time","weather","temperature" don't come out in sentence with sore, are, kore. However, it doesn't mean it's been omitted but there's always no "sore,are,kore". So, 午後ごじです。is a complete sentence. Its neither rude or childish or grammatically wrong, though you might feel strange because there's no subjects and verbs. If you put something to make sentence longer, what about putting "今（いま）"　at the beginning? 今、午後ごじです。 -　ima, gogo goji desu. -- 5PM now.

Hi! mario. Incidentally,am is called "gozen ( 午前 )".