Question about the unit of IntervalDayTime

[zhenghaozzz]

In Volex, the unit of IntervalDayTime is millisecond.

struct IntervalDayTime {
  // ...
 private:
  /// Number of milliseconds in the interval.
  int64_t milliseconds_;
};

However, the unit of Timestamp is nanosecond.

struct Timestamp {
  // ...
 private:
  int64_t seconds_;
  uint64_t nanos_;
};

The precision of IntervalDayTime is far less than Timestamp. I want to know why millisecond is chosen for IntervalDayTime.