http://forums.dds6qkxpwdeubwucdiaord2xgbbeyds25rbsgr73tbfpqpt4a6vjwsyd.onion/t/dealing-with-file-system-timestamps/10427
A lot of filesystems also store a “created” time field, which will not be affected by opening/saving. Also, some file formats will actually leak additional info about what software opened them if you open/resave them. So I wouldn’t really recommend that approach. I think there exist tools that will wipe filesystem timestamp metadata; if such tools exist (I haven’t looked very carefully) then they’re probably preferable.