The big question would be... what do you want for the final file names for the file names above? I could guess on some of the more obvious ones but I hate guessing at things like this.

Speaking of "naming conventions"... based on what you've requested, there are two problems here...

1. The tail is wagging the dog. These files should be named following both a format and naming convention at the source instead of you having to write code to handle such things on the backend.

2. You still don't have an actual naming convention because even the modified date can appear anywhere in the file name or could be totally missing as in that last example above.

My recommendation would be to drop this task and use the file creation date, instead.