1. A computer-implemented method for resolving ambiguities in date values associated with an attribute of an entity, the method comprising: at a computer system including one or more processors and memory storing one or more programs, the one or more processors executing the one or more programs to perform the operations of: obtaining a first text string associated with an attribute of an entity, wherein the first text string is extracted from a first web document; determining that the first text string conforms to one or more date formats; assigning a first confidence value for each of the date formats for the first text string based on a first number of unknown variables that remain when interpreting the first text string using each of the date formats; obtaining a second text string associated with the attribute of the entity, wherein the second text string is extracted from a second web document; determining that the second text string conforms to one or more of the date formats; assigning a second confidence value for each of the date formats for the second text string based on a second number of unknown variables that remain when interpreting the second text string using each of the date formats; determining a first date string expressed in a date format with a highest first confidence value for the first text string; determining a second date string expressed in a date format with a highest second confidence value for the second text string; and merging a first subset of the first date string and a second subset of the second date string to obtain a date value for the attribute.