The default start date of your electricity bill is from the 1st of each month. But many electricity companies charge you from the other day of each month. Now in Iammeter system, you can set the billing date so the system will calculate the monthly electricity bill based on the start date you set.
Login your Iammeter account, go to "My Places" and click "Edit" button.
Go to "Billing day" and set the actual billing date.
After setting it, the electricity bill will be dated from the date you set. For example, if you set it as "15". The bill is dated from 15th this month to 14th next month.