Your flow file is not posted correctly and is corrupt
In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```
)
```
code goes here
```
You can edit and correct your post by clicking the pencil icon.
See this post for more details - How to share code or flow json
You JSON file is not a JSON and all but useless.
There’s a great page in the docs (Working with messages : Node-RED) that will explain how to use the debug panel to find the right path/value for any data item.
Pay particular attention to the part about the buttons that appear under your mouse pointer when you over hover a debug message property in the sidebar.
To the issue
const diffMs = sleepEnd - sleepStart;
sleepEnd and sleepStart are still time objects, you have not create variables that contain the getime() integers. Try creating two new vars using sleepEnd.getTime(), and use these vars for your calculation.