Let's say we have a web server. Server startup time is the time the server process was started.
Say, the startup time was being leaked via its response headers due to the carelessness of a programmer by not getting current time while the generation of the response. The programmer is using the time variable he initialized before entering the "listening" loop, which remains the same throughout the process's lifetime, which is the time the process was started.
As we know that date field must be the date and time the response was generated which it is not in the current situation.
Could this information be useful to an attacker? How?