In order to get your accounting correct you have to set the number of months in your present fiscal year. You do this under Bookkeping → Bookkeeping Settings when logged into Billogram.
When registering your company with the Swedish Companies Registration Office you choose which type of fiscal year your company will have.
The first day of your fiscal year starts the day your company is registered and will therefore not add up, allowing the first year to be up to 18 months long.
Fiscal years not following the calendar yeat (1 January - 31 December) are called broken fiscal years and begin the first day of any month.