Even if you say you timed it, I would suggest to check your career report in MCC. There you can find your total campaign time.
Mastervic118
Recruit - Iron
Halo 5: Guardians
Spartan Company
Forum Activity
All Badges
Feet First
Made your first hot drop into HaloWaypoint.com
Statistician
View your Service Record on the web.
Enlisted
Become a member of an active Spartan Company.