The number of digits places the era first. The very first holograms, around 1984–1986, carry 6 digits. From 1986 to about 2005, numbers have 7 digits: the first digit gives the series, 0 to 9. From 2005 to 2021–2022, they have 8 digits: the first two give the series, 10 to 32.
Each series corresponds to a production period, listed in the table on this page. These periods remain approximate: series changed during production, at an irregular pace. Series 11 alone covers 2006 to 2008, while the year 2019 spans series 27, 28 and 29. This is why the checker gives you a production period, not an exact date. Depending on the reference, the periods for series 25, 27, 28, 30 and 31 vary by about a year.