Calculate daily means from 5-minute interval data

The fact that your projects are in a single time zone is irrelevant. I am
not sure how you can be so confident in saying it does not matter whether
the data were recorded in PDT or PST, since if it were recorded in PDT
then there would be a day in March with 23 hours and another day in
November with 25 hours, but if it were recorded in PST then there would
always be 24 hours in every day, and R almost always assumes daylight
savings if you don't tell it otherwise!