The dates, that are mentioned in the list on the Indexroom page, are the dates for the first print. When two years are mentioned, like 1909/1912, then the first print was made somewhere in that period. Most decks were produced for many years, which makes it sometimes hard to date.
Help can be found in the size of the indicators, the size of the cards, the used trademark logo on the cards or box, a tax stamp or the used backside and joker.
The initial decks were published by the "Nederlandsche Speelkaartenfabriek" (NSF). By the end of 1912 the company's name was changed to "Speelkaartenfabriek Nederland" (SN). In products there is not such a distinct transition. Both the machinery and the lithographic stones were taken over by SN, aswell as the stock. Decks in SN boxes or wrappers may well have been made by the NSF or printed with the same lithographic stones. Trademark logo nr. 2 was used by both companies. So in some cases both NSF and SN are stated as publisher.
Remember that in general playing cards are printed and put in stock. It may take many years before that stock is sold. So it is very well possible that a deck was printed earlier than a tax stamp or used trademark may suggest.
First there are the indicators (H,V,B, or K,Q,J).
When you find black indicators on the courts and aces of the diamonds and hearts, it is safe to assume that the deck was made by NSF, which dates it 1909/1912. When there are red indicators on these courts and aces it's made by SN. In general decks with small indices are older than the ones with large indices. But for more exact dating other info will have to be used.
.......................(This logo was also used by SN after 1912 until ca. 1925)
And last but not least, the printing technique that was used may give an indication to the age of the deck. Roughly speaking chromolithography was used by SN until ca. 1940. After the war offset (and occasionally bookprint) was used. So a magnifying glass is always handy to take along.