Invoice numbers are structured as follows:
YYMMDDNN
Example:
240206011: the year (here 2024)
240206011: the month (here February)
240206011: the day (here the 6th)
240206011: consecutive invoice number for this day (here the 11th invoice)
You can therefore use the invoice number to determine exactly when the invoice was issued.
Are invoice numbers structured this way permitted?
Yes, of course: the invoice numbers are ascending. Gaps may occur as long as the logic is understandable.
See the German VAT Act (UStG), Section 14, paragraph 4, item 4:
[…] a consecutive number consisting of one or more series of numbers, assigned once by the invoice issuer to identify the invoice (invoice number),
VAT Application Decree (as of 12/2016):
“An uninterrupted sequence of invoice numbers issued is not mandatory”
“A combination of numbers and letters is also possible.”
It is left to the invoice issuer “to decide how many and which separate numbering sequences are created, within each of which an invoice number is assigned only once.”
“The individual numbering sequences do not necessarily have to be uninterrupted.”