The "System clock has been set back" variation of the "Invalid or missing license file" errors usually occurs when the computer’s system clock has been set forward and later corrected, but it could also be caused by a file with an incorrect time stamp on the computer. FLEXlm (the licensing software used by Maple) is throwing an error because it interprets the presence of future-dated files as an attempt to circumvent the license expiration date.
There are likely files present on your machine that have time-stamps that are in the future. These files may cause problems if you have to re-install Maple, and may cause conflicts in other programs as well.
After correcting the system clock (if necessary), you may want to search for these files, then re-save them so that their time-stamps are the present time. Also, please ensure that the files in question are not system-critical files before modifying them. You must do this for all future-time-stamped files, so it may not be very easy to do, but it should solve the issue.