Knowledge can be relatively stable (basic math, anatomy, taxonomy, physical geography, etc.) and highly volatile (economic indicators, high-tech knowledge, personal statistics, etc.). It is important that you provide your items with time stamping or other tags indicating the degree of obsolescence. In case of statistical figures, you might stamp them with the year they have been collected. When learning software applications, it is enough you stamp the item with the software version. Once you have newer figures you can update your items. Unfortunately, in most cases you will have to re-memorize knowledge that became outdated. Date stamping is useful in editing and verifying your knowledge; however, you will rarely want to memorize stamping itself. If you would like to remember the changes of a given figure in time (e.g. GNP figures over a number of years), the date stamping becomes the learned knowledge itself.