The issue with just using UTC is that the date changes in the middle of the day. Like in Seattle it would change from one day to the next at what is currently 4 PM.

You try to plan an event for the 16th, and which physical day it's in depends on whether it's before or after an (ultimately arbitrary) cutoff time. You say "oh this happened yesterday" well was it a few hours ago before the date change or do you mean the previous physical day.

Also weekdays would be messed up. You work "Monday to Friday" between the current 9 AM and 5 PM, but then how does that work when Monday starts at (what's currently) 4 PM? Do you work between 4 and 5 since it's during work hours on a Monday? And on Fridays do you stop working at 4 because after that it becomes Saturday? You say you're busy all day Wednesday, but does that mean you're suddenly available after 4 PM when the date changes?