There are public users and owner users. Specifically, you don't trust input from public users. You absolutely should trust input from owner users, or if you won't by default for some reason, you absolutely should give them the option to do so. Primarily, you should be giving them the choice as to what their system will do when it cannot reliably determine the actual time.
The alternative is to pretend that you can somehow divine a 'projected secure time.' Just, the gall to even write a sentence like that.
The alternative is to pretend that you can somehow divine a 'projected secure time.' Just, the gall to even write a sentence like that.