What does it mean when your analytics report is telling you that a visit to your website was of a duration of zero seconds?

It's all because of the way Google tracks visitor behaviour.

Google Analytics tracks visits to the pages on your site by using cookies triggered by code on every page being tracked.

When a visitor goes to a page, it triggers this code and Google registers it as a visit, along with other data such as the time they arrived there.

If they go to another page on your site, the code on that page will trigger another record, and the time that the visitor arrived at the next page – which is effectively the time they left the first one.

The difference between the two events (the time they arrived and the time they left) is used to calculate the time spent on that first page.

But if the visitor only visited one page then left the site then there is no second event for Google Analytics to record and use to calculate the time difference so, single page visits can show up as having 'zero' duration.

This article "Time On Site & Bounce Rate: Get the real numbers in Google Analytics" includes a script snippet to add to your site if want to resolve this issue.