If your meter bills overlap by one day (last day of one = first day of next), then the second month gets credit for the day.
When you enter your last bill, Portfolio Manager will look at the previous bill to see if there was an overlap and if there was, it won't count the last day of that last month.
It is very common to be billed this way and is the default pre-population in Portfolio Manager when entering meter bills. However, Portfolio Manager only allows this 1 day overlap. Anything more will be flagged as an error.