日時
にちじ · nichiji · JLPT N2 · noun
time
📅🕒 日 (day) + 時 (time) = 日時: day plus time, so date and time
日 means day and 時 means time, so 日時 simply stacks the two in order: day first, then time. Think of it as a date followed by a clock reading.
日 — pictograph of the sun
日 is a pictograph of the sun, originally drawn as a circle with a dot or stroke inside representing the sun's substance. Over time the round shape was squared off into the modern rectangular form. It means 'sun' and by extension 'day'.