Structure
YY.MM.DD-SSS.CC: birth date, a sequence number (odd for men, even for women) and two check digits.
The check is 97 − (first 9 digits mod 97); for people born in 2000 or later a 2 is put in front of the 9 digits before calculating.
That's how the tool tells 1917 from 2017.
BIS numbers (for people not in the national register, such as foreign workers) add 20 to the month when the sex was unknown at registration and 40 when it was known.
A month or day of 00 means the date is partly unknown.
Privacy
A national number is personal data. This page processes it only in your browser and never logs or transmits it. Only validate numbers you are allowed to process, and use fictional test data in development.