…时【shí】

时 after a verb phrase means "when" or "while". It does the job of 的时候 in writing, and it attaches directly, with no 的 in between.

The 时 clause comes first and the main clause follows, the same order as 的时候. English can put "when" at either end; Chinese can't.

• × 请关灯离开时 → 离开时请关灯

Written, not chat

Signs, instructions, news and essays use 时. Between friends, 的时候 is normal and 时 sounds stiff.

当…时

当 in front adds a formal "at the moment when". The clause between them can be long.

时 is not a word for "time"

On its own 时 is no noun. "Time" is 时间, and "hour" is 小时.

• × 我今天没有时 → 我今天没有时间