"It is 6:40."
Rokuji yonjyuppun desu. ろくじ よんじゅっぷん です。 Rokuji yonjippun desu. ろくじ よんじっぷん です。
です is generally at the end of most japanese sentences, sometimes it can be dropped or replaced for more colliquial purpose or other reasons. Think of it as "is" for some situations, sometimes the subject particle (は) can be thought of as "is" as well to make is easier. この車は大きいです。This sentence, the "is" can be connected to the は, but 十一時です, the は can be connected with the です since the subject is dropped (this could be a responce to 今、 何時ですか。).