Requirements:

• Steam (duh)
• The launcher and/or game added as a Non Steam game.
  As far as I know this trick only works on Windows and Linux, this guide will only assume youre using Windows.

If you need help on that, checkout Step 3 in my Controller guide, https://forum.plutonium.pw/post/135318

Step 1: Downloading Millennium

• Download the client! This client hooks into steam to allow you to have themes and plugins to the software! No worries! Its completely safe! (Even where Valve directly mentions it in their docs!) The setup is pretty straight forward! All you need to do is read through it and youre all set!

Download link: https://steambrew.app

Step 2: Downloading Non Steam Playtime

• Once you install the client, click on the Millennium tab when clicking Steam.
  
• Click on Plugins and then Load a Plugin
• It will then ask for an ID, paste this

02bed50d10a8

• If for some reason it doesnt work, check out its plugin page. https://steambrew.app/plugin?id=02bed50d10a8
• Then Enable it!

Step 3: Setting it up to track your playtime!

Downside of this, is it doesnt include any past playtime, thats up to you to really define, you can have it restart to 0, try to be as accurate as possible, or be a cheater and make yourself have the playtime of a 1 million hours!

• Right click the game you want to track. Then Select properties.
  
• Select the Play Time tab and then set the time to a time! It cant be 0! It has to be a #h #m #s format.
  • For Example, 1s
    

And presto! Your playtime can finally be tracked! Sadly as far as I know it doesnt carry over platforms but there is a way to transfer the time if youre moving from pc to pc.

For any help, go to the respective support from Millennium, as my support will be limiting.

(Thanks to Resxt for showing me this client)