The JRE relies on periodic checks with an Oracle Server to determine if it (the JRE)is still considered up-to-date with all the available security fixes (above the security baseline). In the past, if the JRE was unable to contact the Oracle Server, it continued to behave as though it is still the most recent version with regard to security, for an indefinite period.
To avoid this problem, a secondary mechanism, that does not rely on external communication, has been added to the JDK 7u10. From this release onwards, all JREs will contain a hard-coded expiration date. The expiration date is calculated to end after the scheduled release of the next Critical Patch Update.
This means that JREs that are unable to contact Oracle Servers for an extended period of time, will now start offering additional protection after a reasonable period, and will not continue to behave as if they were still up-to-date with security fixes.