フリー問題

問題文

Devices that timestamp their own readings must therefore hold a usable clock. Their clocks drift by several seconds a day. Which arrangement keeps their timestamps usable over years of operation?

選択肢

1. The timestamps stay usable through periodic correction from a common source.
2. The timestamps stay usable through later adjustment from a platform estimate.
3. The timestamps stay usable through precise oscillators from a careful manufacture.
4. The timestamps stay usable through elapsed counting from a device start.