新增于: v12.10.0
仅在 bigint: true 传入到生成对象的方法中时出现。
指示最后一次修改此文件的时间戳，以 POSIX Epoch 以来的纳秒数表示。
Added in: v12.10.0
Only present when bigint: true is passed into the method that generates
the object.
The timestamp indicating the last time this file was modified expressed in
nanoseconds since the POSIX Epoch.