Time!

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Across
  1. 2. Short-lived, existing only briefly. (Mr. Smith's favourite English word!)
  2. 4. Lasting only a short time; passing quickly.
  3. 5. Happening at the same time.
Down
  1. 1. Continuing for a long period; often used with illness.
  2. 3. Infinite or endless time.
  3. 4. Related to worldly time, not spiritual.