There are three dates associated with any “public” web page that’s available on the Internet:
#1. The publication date - this is the date when an article or web page is first uploaded on to a public website where humans and search engines can find and access that page.
#2. The indexed date - this is the date when search engine spiders first discover a web page on the Internet. Given the fact that Google has become so good at crawling fresh content, the date of first-crawl is often the same as the actual publication date (#1).
#3. The cache date - this is the date when a web page was last crawled by the Googlebot. Search engines often re-crawl web pages every few days or weeks, sometimes multiple times in a day like for news website, to check if the content has changed.
In the case of news articles, the publishing date of the web page is included but in situations where the date is not specified (or you think the mentioned date in incorrect), you can use a simple Google hack to know when a web page or web domain was last published on the Internet.
Google can tell the date when a web page was first published on the Web.
Step 1. Go to google.com and copy-paste the full URL of any web page in the search box with the inurl: operator (e.g. inurl:www.example.com). Hit enter.
Step 2. Now go to browser address bar (Ctrl+L in Firefox & Chrome or Alt+D in Internet Explorer) and copy-paste
&as_qdr=y15 at the end of the Google search URL. Press enter again.
Step 3. Google will load the search results again but this time, you’ll see the actual publication date of the web page next to the title in Google search results as in the above screenshot.
Because Google can crawl the page the moment it is published on the Internet, the indexed date appearing in search results is often accurate. However, if the content of a web page were changed after the first Google crawl, the publishing date may actually represet the date when it was most recently edited and not the date when it was first indexed or published.
See some more tools to know everything about a website.