Your not being a pest. Let's just get it sorted :-)
The issue is you have VALUES(calendar[date]). Look back at my formula - it uses DAY, not DATE. the Day column in your calendar table should have Sun, Mon, Tues etc
Give me 5 mins and I will post a sample workbook for you
Nothing like a demo workbook to flush out an error
LatestStart = MAXX(values('Calendar'[Day]),CALCULATE(min(Data[Start Time])))
I missed the CALCULATE - sorry.
Here is the workbook demo https://www.dropbox.com/s/u6o1p0rpzuwnl4y/latest%20start.pbix?dl=0