Unfortunately the ChatGPT extension within Shortcuts is super buggy and claims you're logged out — so I dumped the raw data from the Screen Time usage step, which looks like this:

Safari (2h 30m), Terminal (54m), Slack (39m), Reddit (28m), Messages (25m), Maps (16m)

to a JSON endpoint, and then used a simple regex to extract all the time values and sum them, and compute screen time for the day from there. Regexes > LLMs!

I am still having some oddities where it combines multiple device screen time, despite only asking for iPhone usage.