How will you be recording the gameplay if you don't mind me asking? I believe you could use a stopwatch as long as it is recorded along side the gameplay and done in real time. Moderators do retime things if they think that your time was off, it just makes their life easier if you can at least put in the description the time of error if that's the only way you can do it.

All you need to do is record video of your run with a timer on screen that records the real time for whichever category it is for. When the run is recorded and uploaded to some form of video hosting site such as Youtube or Twitch, click on the category you ran and hit summit run. Usually you just have to list time you got on the run and give the mods a link to the video do that they can authenticate the run. Just make sure that you read the rules of the category you're running if you haven't already so that you don't make a mistakes :)