Google’s US patent US7346839 B2, “Information retrieval based on historical data,” states that Google may use the length of the domain name registration to distinguish between illegitimate and legitimate domains.
Certain signals may be used to distinguish between illegitimate and legitimate domains. For example, domains can be renewed up to a period of 10 years. Valuable (legitimate) domains are often paid for several years in advance, while doorway (illegitimate) domains rarely are used for more than a year. Therefore, the date when a domain expires in the future can be used as a factor in predicting the legitimacy of a domain and, thus, the documents associated therewith.