My bills don't follow any fixed pattern other than they are every two months. My next bill is due this month. Previous billing periods and billing dates were:
end 15/05/15 billed 18/05/15
end 14/07/15 billed 16/07/15
end 17/09/15 billed 22/09/15
end 05/11/15 billed 12/11/15
end 19/01/16 billed 20/01/16
end 28/03/16 billed 29/03/16