If this question is in line with the topic about ledgers. Then, account number 13 tells you that that account
A.) IS THE 13TH ACCOUNT IN THE LEDGER.
Usually, First account int he third division is presented as 3.1 or 3-1
Third account in the first division 1.3 or 1-3.
First number is the division number and second number is the account number within the division.
posting as 13th day of the month is not posted as an account number. It should be posted as a date on any preferred date format.