If they are translated literally it gives "today is" and "tonight is".
Is it some sentence that got shortened ?
(Also not sure how to classify this question so forgive me if I used the wrong tags...)
They do appear to be shortenings, but perhaps not of any particular wording. 大辞林 says they're short for sentences like the following:
In each case, 大辞林 marks the sentence in quotes with など, implying it doesn't necessarily come from those sentences specifically, but sentences such as the above. The Iwanami Kokugo Jiten does the same thing; when I looked up these words, I found the following examples in 「」 quotes:
In each case, the dictionary marks the sentences with など to illustrate that they're the sorts of sentences that could complete こんにちは or こんばんは, not that those specific words are implied by the greetings.