iPhone 6 current period not matching with the last reset date
The "Current Period" time listed under iOS -> Settings -> Cellular only shows the amount of call time spent, or time you have used the phone, and not how long data has been in use for. Data only shows usage and does not reflect the amount of time. You'll have to mark when you reset the statistics to get an accurate count.

An easy way to do this (one I use often) would be to set a reminder via Siri to "remind me in 48 hours to check data usage" - this way you'll get a reminder near exactly when you want to check.

Thanks!