outlook calendar events sorted by time

You're very close but you're referencing DateTime incorrectly. The
proper format is {parent}/{child}. These will work:

https://graph.microsoft.com/v1.0/me/events?$orderby=start/dateTime
https://graph.microsoft.com/v1.0/me/events?$orderby=start/dateTime desc

https://stackoverflow.com/questions/47331090/sort-events-by-start-date