1. Verify that the modtime option in your Perforce client spec is not checked. Detail explanation is as follows:
| Mod time set to depot submission time: (modtime) | What should the modification time on files in your workspace be set to when copied in by the sync command? By default, the modification time of unopened files on your workspace files shows when they were synced. Choosing this option will cause sync to set each file's modification time to the timestamp associated with it when it was submitted to the depot. |
If checking on this option, for team working together between US and China, strange time stamp will be got. So avoid using it.