こんにちは。

Konnichi wa.

Hello / good afternoon.

Word for word

“As for today…”

Politeness: Neutral

Fixed expression that suits almost everyone, with no politeness marker to adjust.

When you would say it

In daylight hours, to people you do not live with.

Worth knowing

Originally the start of a longer sentence (今日はご機嫌いかが). Between close friends it often disappears; you do not say it to family at home.