What does the error message "system clock has been set back" mean?

Problem description: 

I am receiving this license error message and cannot use PLECS. Why?

Solution: 

This error is raised by the FlexNet licensing module built into PLECS when it detects inconsistencies in the date settings on your computer and you are using a license with an expiry date.

The reason is that such inconsistencies often indicate an attempt to use a non-permanent license beyond its expiration date. The following link gives more information about the problem and ways to resolve it: https://www.geoslope.support/kb/article/97-error-88-309-geostudio-system-clock-has-been-set-back/