The invoice numbers are structured as follows:
YYYMMDDNN
Example:
190206011: the year (here 2019)
190206011: the month (here February)
190206011: the day (here the 6th)
190206011: consecutive invoice number on this day (here the 11th invoice)
Thus one can trace exactly on the basis the invoice number when the invoice was written.
Are the invoice numbers even allowed?
Yes, of course, because the invoice numbers are ascending. Gaps may occur as long as the logic is comprehensible.