Metadata Help: Static and Dynamic Pages
Web pages are either static or dynamic.
"Static" pages are "pre-built" or "as is"--what one thinks of as Web 1.0 content. An author created an html page, saved it, waiting for you to open it. What you see is what the author saved. Standard HTML pages are static Web pages. HTML code defines the structure and content of the page. Every time you open a static page it is the same, unless an author or developer updates and publishes a new file. This is where the "Last-Modified" date originates.
"Dynamic" Web 2.0 pages have content that changes or is created "on-the-fly" as you open it. When pages with extensions such as PHP, ASP, and JSP (server-side codes) are opened, the server is permitted to generate unique content. A common example is a display of the current time and date. If you see the current date and time, you are looking at a dynamic page. Other examples include unique responses based on input by a user and information pulled from a database based on user preferences (like search results).
Finding a date for the original content of a dynamic page cannot be determined by looking for Last-Modified metadata--in fact, such data does not exist. If you look for publication date metadata (Date) you will always get the current server time: the instant the server created the page. Other investigative methods must be used to determine or estimate the publication date of dynamic pages: comparing similar content from multiple sites, looking at the URL for clues and checking web archives (archive.org) to see how long the content has remained the same.
Credits: this information is adapted from What are static and dynamic Web pages? by Per Christensson