In a mobile computing system, caching data items at the mobile clients is important to reduce the data access delay in an unreliable and low bandwidth mobile network. However, efficient methods must be used to ensure the coherence between the cached items and the data items at the database server. By exploring the real time properties of the data items, we propose a cache invalidation scheme called: Invalidation by Absolute Validity Interval (IAVI). We define an absolute validate interval (AVI) for each data item based on its real time property, e.g. update interval. A mobile client can verify the validity of a cached item by comparing the last update time and its AVI. A cached item is invalidated if the current time is greater than the last update time by its AVI. With this self-invalidation mechanism, the IAVI scheme uses the invalidation report to inform the mobile clients about the change of AVI rather than the update event of the data item. As a result, the size of invalidation report can be reduced significantly. Performance studies show that the IAVI scheme can significantly reduce the mean response time and invalidation report size under various system parameters.