After (exactly) 5-months, my cellular data usage logs reset to zero. What happened? Starting on 06/26/2017, at the end of each day, I began logging the cellular data usage for each of my Apps and System Services in a spreadsheet. On 06/26 all the cellular usage was at 0-MB. On 11/26 the data was as such:  

/Settings/Cellular/
} Google (994 MB)
} Pokemon Go (625 MB)
} Uber (2754 MB)
...

/Settings/Cellular/System Services
} iTunes Acct. (115 MB)
} Messaging Services (241 MB)
} DNS Services (32.3 MB)
...

On 11/27 (yesterday), I started to input my cellular usage into my spreadsheet, and both Apps, and System Services, had reset to zero!!!

I did not do anything unusual to the iPhone.
What could have caused this "reset to zero" of my cellular usage logs (coincidentally) exactly 5 months after I started logging the usage?


  I guess the logs can't go to infinity.
  Actually, is this how cellular data logging is suppose to work on an iPhone?


thanks.
Go to Settings > Cellular and keep swiping up until you get to the very bottom of the page. You'll notice a blue Reset Statistics option. Below this check the Last Reset date to see if it coincides with when your data went to zero.


If it does, you have your answer. It's likely you've accidentally tapped on Reset Statistics, or that perhaps someone else did.
If it doesn't, then it's impossible for anyone here to give you a definitive answer as to what happened.


As for how it's supposed to work, there's no time limit as such. One of my devices shows it was last reset on 19 September 2013!