Error when trying to create Start Times

I am a getting an error trying to create start fields. I have 3 start times that are chronologic

Tech Start>Patient In>Pro Start

Tech Start is a random time

Patient In is a formula (This works):
Time.at(field(“Tech Start”) + (random(0,120)).minutes).utc.strftime("%I:%M %p")

Pro Start is a formula (This does NOT work):
Time.at(field(“Patient In”) + (random(0,120)).minutes).utc.strftime("%I:%M %p")

This is the error.
error: no implicit conversion of ActiveSupport::Duration into String