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?


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/