A question often arises regarding the Last Modified date/time value in Internet cache records. The value is derived from the repsponse provided by the server as part of the HTTP (Hypertext Transfer Protocol) process.
HTTP Response
When a Web Browser requests a page or object from a Web Server, it does so by sending an HTTP request. After receiving and interpreting a request message, the server responds with an HTTP response message (as shown in Figure 1). The HTTP response message is comprised of two parts:
- HTTP Header
- Data
In Figure 1 below, line numbers 1 to 12 contains the HTTP Response Header. There are a number of different possible date/time values that can be returned. In this case, we have a Last Modified timestamp. It is this timestamp that the Web Browser uses in cache management process. The actual data commences at line 15 (line 14 contains the data length).
Figure 1
Date Last Modified [UTC]
This date is a timestamp (in UTC) which reflects the date and time the webpage (or object) was last modified (last written). This information is passed back to the browser as part of the HTTP response. Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time.