The default filename is in the following file naming format:
YYYY-MM-DD HH-MM-SS.[file extension]. Eg. 2020-03-06 14-05-32.mp4
It is time-stamp based on when you started recording the video. If you wish to change this, you can do so within OBS:
- Click Settings
- Click Advanced
- In the section titled Recording, please update the naming convention in the Filename Formatting field.