Is there a way to find out how much time has passed since a web page was changed?
For example,
I have a page hosted at: www.mywebsitenotupdated.com
For checking the Last Modified header, you can use httpie (docs).
pip install httpie --user
$ http -h https://martin-thoma.com/author/martin-thoma/ | grep 'Last-Modified\|Date'
Date: Fri, 06 Jan 2017 10:06:43 GMT
Last-Modified: Fri, 06 Jan 2017 07:42:34 GMT
The Date is important as this reports the server time, not your local time. Also, not every server sends Last-Modified (e.g. superuser seems not to do it).