1. A method of providing data entities of a database with a time dependent value for an attribute of the entity, the method comprising: i) having values of attributes of said entities in said database; ii) for said entities providing a valid time start time associated with each said value of said attribute; iii) for said entities providing a valid time end time associated with each said value of said attributes; iv) for said entities providing a transaction time associated with said valid time start time and said valid time end time for each said value; whereby v) when querying said database an appropriate said transaction time is used so as to ensure that an appropriate value for said attribute is used and that appropriate valid time start and end times are used.