No I'm sorry. This is a cop out. You are a professional reviewer paid to play games. You should have enough affinity with a gaming experience to realise when the timer of a game doesn't reflect the total amount of time playing a game.
I will accept that excuse from some of the people in here, especially ones unfamiliar with the genre, but not from reviewers. It's lazy.
What I'm saying is, in my case, I would be shocked if what my timer read—5:04—was far off from what I put into the game. I played and beat it in an afternoon, and while I don't remember exactly my start/stop times, I do know it was definitely a length where five hours made sense.
If it were a case where I had played eight hours of the game, my timer read five, and I went, "Well, that must be right," I'd totally agree with you. That wasn't the case, at least for me.
However, I definitely agree that if you are going to review a game and point out the game's length in your review, you should have a more accurate system for keeping track than just an in-game timer. (Because, really, games are so unreliable in such options being available.)